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No, and doing so would lead to serious balance issues. Most spells with long casting times have them specifically because they are very powerful. The only way for this to work is if they took time to prepare and cast the spells in the morning, and then immediately went into the battle, or, at least, did so while the spells were still active.

Consider the spell Prayer of Healing:

Prayer of Healing

2nd-level evocation

Casting Time: 10 minutes Range: 30 feet Components: V Duration: Instantaneous

Up to six creatures of your choice that you can see within range each regain hit points equal to 2d8 + your spellcasting ability modifier. This spell has no effect on undead or constructs. At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the healing increases by 1d8 for each slot level above 2nd.

This spell takes ten minutes to cast specifically because it does a lot of healing to a lot of people, without requiring touch, at a very early level. If castable during combat, every encounter would become incredibly trivial.

Keep in mind that each round in combat is 6 seconds of in-game time. This spell is completely uncastable in combat, and for good reason.

Consider also Conjure Elemental:

Conjure Elemental

5th-level conjuration

Casting Time: 1 minute Range: 90 feet Components: V, S, M (burning incense for air, soft clay for earth, sulfur and phosphorus for fire, or water and sand for water) Duration: Concentration, up to 1 hour

You call forth an elemental servant. Choose an area of air, earth, fire, or water that fills a 10-foot cube within range. An elemental of challenge rating 5 or lower appropriate to the area you chose appears in an unoccupied space within 10 feet of it.

This is marginally more castable in combat, but just barely, requiring 10 full rounds of doing nothing else but summon the elemental. This is because it adds an entire new creature to the field, and lasts for an hour.

So, no, this is not an option. Not with feats, not by default, and it should really not be done by house ruling, because it will very thoroughly break combat if there are any spellcasters present.

However, there is the option to Ready a Spell, but this must be done during combat, and must be a spell that you could feasibly cast on your turn. This is the same as Readying an Action, and means that you specify the spell you wish to cast and the condition under which you will cast it. If that condition does not arise before your next turn, the spell slot is lost, but the spell is not cast. This is the only way to prepare a spell before using it, and only works for one turn in combat.

I believe some of the confusion here may arise from the usage of "prepare" used in the book for certain spellcaster classes. Some caster classes can always use all the spells they know in a certain day; others must pick and choose after each long rest which spells they wish to be able to cast that day. This simply means that the prepared spells are on the list of spells that the caster can use that day; casting times are still the same.

No, and doing so would lead to serious balance issues. Most spells with long casting times have them specifically because they are very powerful. The only way for this to work is if they took time to prepare and cast the spells in the morning, and then immediately went into the battle, or, at least, did so while the spells were still active.

Consider the spell Prayer of Healing:

Prayer of Healing

2nd-level evocation

Casting Time: 10 minutes Range: 30 feet Components: V Duration: Instantaneous

Up to six creatures of your choice that you can see within range each regain hit points equal to 2d8 + your spellcasting ability modifier. This spell has no effect on undead or constructs. At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the healing increases by 1d8 for each slot level above 2nd.

This spell takes ten minutes to cast specifically because it does a lot of healing to a lot of people, without requiring touch, at a very early level. If castable during combat, every encounter would become incredibly trivial.

Keep in mind that each round in combat is 6 seconds of in-game time. This spell is completely uncastable in combat, and for good reason.

Consider also Conjure Elemental:

Conjure Elemental

5th-level conjuration

Casting Time: 1 minute Range: 90 feet Components: V, S, M (burning incense for air, soft clay for earth, sulfur and phosphorus for fire, or water and sand for water) Duration: Concentration, up to 1 hour

You call forth an elemental servant. Choose an area of air, earth, fire, or water that fills a 10-foot cube within range. An elemental of challenge rating 5 or lower appropriate to the area you chose appears in an unoccupied space within 10 feet of it.

This is marginally more castable in combat, but just barely, requiring 10 full rounds of doing nothing else but summon the elemental. This is because it adds an entire new creature to the field, and lasts for an hour.

So, no, this is not an option. Not with feats, not by default, and it should really not be done by house ruling, because it will very thoroughly break combat if there are any spellcasters present.

No, and doing so would lead to serious balance issues. Most spells with long casting times have them specifically because they are very powerful. The only way for this to work is if they took time to prepare and cast the spells in the morning, and then immediately went into the battle, or, at least, did so while the spells were still active.

Consider the spell Prayer of Healing:

Prayer of Healing

2nd-level evocation

Casting Time: 10 minutes Range: 30 feet Components: V Duration: Instantaneous

Up to six creatures of your choice that you can see within range each regain hit points equal to 2d8 + your spellcasting ability modifier. This spell has no effect on undead or constructs. At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the healing increases by 1d8 for each slot level above 2nd.

This spell takes ten minutes to cast specifically because it does a lot of healing to a lot of people, without requiring touch, at a very early level. If castable during combat, every encounter would become incredibly trivial.

Keep in mind that each round in combat is 6 seconds of in-game time. This spell is completely uncastable in combat, and for good reason.

Consider also Conjure Elemental:

Conjure Elemental

5th-level conjuration

Casting Time: 1 minute Range: 90 feet Components: V, S, M (burning incense for air, soft clay for earth, sulfur and phosphorus for fire, or water and sand for water) Duration: Concentration, up to 1 hour

You call forth an elemental servant. Choose an area of air, earth, fire, or water that fills a 10-foot cube within range. An elemental of challenge rating 5 or lower appropriate to the area you chose appears in an unoccupied space within 10 feet of it.

This is marginally more castable in combat, but just barely, requiring 10 full rounds of doing nothing else but summon the elemental. This is because it adds an entire new creature to the field, and lasts for an hour.

So, no, this is not an option. Not with feats, not by default, and it should really not be done by house ruling, because it will very thoroughly break combat if there are any spellcasters present.

However, there is the option to Ready a Spell, but this must be done during combat, and must be a spell that you could feasibly cast on your turn. This is the same as Readying an Action, and means that you specify the spell you wish to cast and the condition under which you will cast it. If that condition does not arise before your next turn, the spell slot is lost, but the spell is not cast. This is the only way to prepare a spell before using it, and only works for one turn in combat.

I believe some of the confusion here may arise from the usage of "prepare" used in the book for certain spellcaster classes. Some caster classes can always use all the spells they know in a certain day; others must pick and choose after each long rest which spells they wish to be able to cast that day. This simply means that the prepared spells are on the list of spells that the caster can use that day; casting times are still the same.

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No, and doing so would lead to serious balance issues. Most spells with long casting times have them specifically because they are very powerful. The only way for this to work is if they took time to prepare and cast the spells in the morning, and then immediately went into the battle, or, at least, did so while the spells were still active.

Consider the spell Prayer of Healing:

Prayer of Healing

2nd-level evocation

Casting Time: 10 minutes Range: 30 feet Components: V Duration: Instantaneous

Up to six creatures of your choice that you can see within range each regain hit points equal to 2d8 + your spellcasting ability modifier. This spell has no effect on undead or constructs. At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the healing increases by 1d8 for each slot level above 2nd.

This spell takes ten minutes to cast specifically because it does a lot of healing to a lot of people, without requiring touch, at a very early level. If castable during combat, every encounter would become incredibly trivial.

Keep in mind that each round in combat is 6 seconds of in-game time. This spell is completely uncastable in combat, and for good reason.

Consider also Conjure Elemental:

Conjure Elemental

5th-level conjuration

Casting Time: 1 minute Range: 90 feet Components: V, S, M (burning incense for air, soft clay for earth, sulfur and phosphorus for fire, or water and sand for water) Duration: Concentration, up to 1 hour

You call forth an elemental servant. Choose an area of air, earth, fire, or water that fills a 10-foot cube within range. An elemental of challenge rating 5 or lower appropriate to the area you chose appears in an unoccupied space within 10 feet of it.

This is marginally more castable in combat, but just barely, requiring 10 full rounds of doing nothing else but summon the elemental. This is because it adds an entire new creature to the field, and lasts for an hour.

So, no, this is not an option. Not with feats, not by default, and it should really not be done by house ruling, because it will very thoroughly break combat if there are any spellcasters present.