An effective, thermal three-carbon + two-carbon cycloaddition for cyclopentenone formation: formal 1,3-dipolar cycloaddition of cyclopropenone ketals

C–C Bond Cleavages of Cyclopropenes: Operating for Selective Ring-Opening Reactions

“…Moreover, the examples covered in this section include thermal uncatalyzed as well as metal-catalyzed cycloadditions. Among types of cyclopropenes, Boger and co-workers demonstrated the utility of cyclopropenone acetals as reagents for [1 + 2], 206 [3 + 2], 207 and [3 + 4] 208 cycloaddition reactions. 209,210 The most exciting illustration of this ability culminated with one of the most concise formal total synthesis of tropolone alkaloid Colchicine (306).…”

“…Among types of cyclopropenes, Boger and co-workers demonstrated the utility of cyclopropenone acetals as reagents for [ 1 + 2], [ 3 + 2], and [ 3 + 4] cycloaddition reactions. , The most exciting illustration of this ability culminated with one of the most concise formal total synthesis of tropolone alkaloid Colchicine ( 306 ) . A strategic intermolecular [3 + 4]-cycloaddition of cyclopropene 103 with Eschemoser’s α-pyrone 304 was relevant to prepare the structural skeleton of this privileged molecule (Scheme A).…”

The [4 + 2] Cycloaddition of 2‐Pyrone in Total Synthesis

“…reaction intermediates. The tropone ring was designed to be introduced by the condensation of pyrone 417 with cyclopropenone ketal 418 . However, after the failure of the thermal [4 + 3] cycloaddition of cyclopropenone ketal 418 that proceeded through a reversibly generated π‐delocalized singlet vinylcarbene intermediate, the pressure‐promoted Diels‐Alder reaction of 417 and 418 was found effective.…”

“…The tropone ring was designed to be introduced by the condensation of pyrone 417 with cyclopropenone ketal 418 . However, after the failure of the thermal [4 + 3] cycloaddition of cyclopropenone ketal 418 that proceeded through a reversibly generated π‐delocalized singlet vinylcarbene intermediate, the pressure‐promoted Diels‐Alder reaction of 417 and 418 was found effective. Under 12—13 kbar pressure at room temperature, the key Diels‐Alder reaction proceeded smoothly to provide a mixture of adduct exo ‐ 419 and granditropone 420 , which was derived from thermally unstable endo [4 + 2] cycloadduct endo ‐ 419 that suffered loss of CO 2 by retro Diels‐Alder reaction and subsequent hydrolysis of the labile ketal upon chromatographic purification.…”

Synthesis of naturally occurring tropones and tropolones

“…263–267 The cycloaddition with α-pyrone is particularly intriguing since it provides a way to access tropone- or tropolone-containing natural products, such as colchicine (Scheme 74). 265 It was believed that the cyclopropenone ketal 74-1 was in equilibrium with the vinylcarbene species 74-2 , which underwent [4+3] cycloaddition with α-pyrone 74-3 to afford intermediate 74-4 with a lactone bridge.…”