Can you respond to your opponent casting a spell during their main phase by casting a sorcery with Wilderness Reclamation in play?

Wilderness Reclamation gives you extra mana by untapping your lands at the beginning of each end step, which means you get a burst of mana during your opponent's end step (or your own). However, this mana boost does not change the fundamental timing rules for casting sorceries.

Under CR 307.1, sorcery spells can only be cast during your own main phase, and only when the stack is empty. When your opponent casts a spell, the stack is not empty — it contains at least that spell — so the condition for casting a sorcery is not met, regardless of how much mana you have available.

Wilderness Reclamation's untap trigger goes on the stack at the beginning of the end step (CR 603.3). While that trigger is on the stack, you still cannot cast a sorcery, because the stack is not empty, and even once it resolves, you are in the end step, not your main phase. The extra mana is best used for instants or activated abilities.

Concrete example: Your opponent is in their main phase and casts Divination. You have Wilderness Reclamation in play with five lands untapped. You cannot cast a sorcery like Cultivate in response — Cultivate requires your own main phase with an empty stack (CR 307.1). You could, however, cast an instant like Counterspell if you had the mana.

In short, Wilderness Reclamation provides additional mana resources but does nothing to grant sorcery-speed spells flash or to change the timing restrictions codified in CR 307.1 and CR 116.1.