Formation of Bicyclic Pyrroles and Furans Through an Enone Allene Photocycloaddition and Fragmentation Sequence

Abstract

The [2+2] photocycloaddition of allenes 15-18, 23 and 26 was studied. Irradiation of a solution of these substrates in acetonitrile at 300 nm resulted in the clean conversion of the starting materials into a mixture of photoproducts. The major product in all cases was a bicyclic pyrrole or furan fused to an eight membered ring (43-70 % yield). The formation of these products is thought to be a result of a heteroatom-induced fragmentation of the straight adduct (7). This is supported by irradiation of the carbon analogue 32 which al-

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

“… 504 The Hiemstra group isolated tert -butoxycarbonyl (Boc)-protected pyrroles such as 287a upon irradiation of enone allenes like 284 ( Scheme 92 ). 505 , 506 Presumably, the [2 + 2] photocycloaddition proceeds in a straight fashion to highly strained intermediate 285 , which cleaves in a retro-Mannich fashion to zwitterion 286 . The corresponding crossed [2 + 2] photocycloaddition products were isolated in yields of 10-20% accounting in part for the apparent deficiencies in the mass balance.…”

Photochemical Approaches to Complex Chemotypes: Applications in Natural Product Synthesis

“… 270 Related photocycloaddition/retro-Mannich sequences have also been reported for 3-aminocyclopentenones 271 and for the synthesis of 6-aza-bicyclo[3,2,1]octan-3-ones 272 and pyrroles. 273 , 274 …”

A Chiral‐Pool‐Based Approach to the Core Structure of (+)‐Hyperforin

“…In this context, functionalization of the C5 methylene carbon of ketone 16 first called for a Saegusa-Ito oxidation [TMSOTf, NEt 3 ; then Pd(OAc) 2 /O 2 ] [13] to afford enone 17 in 75 % yield over the two steps [93 % based on recovered starting material (brsm)]). After extensive optimization of the reaction conditions, conjugate addition of the organocuprate reagent (CuBr·Me 2 S, TMSCl, HMPA) derived from alkynyl halide 8 [14] proceeded smoothly to afford TMS enol ether 18 as a single diastereoisomer (95 % brsm), though the stereochemistry of the newly formed stereocenter at C5 could not be determined at this juncture. However, the stereochemical outcome of this alkylation process was inconsequential due to the subsequent conversion of 18 into enone 20 through initial enol ether hydrolysis (pTsOH) followed by oxidative selenium chemistry (LDA, PhSeBr; then pyridine/H 2 O 2 , 73 % yield over the three steps).…”