Photocycloaddition of Cyclohex‐2‐enones to Penta‐1,2,4‐triene

Abstract

Dedicated to Professor Henning Hopf on the occasion of his 65th birthday Dihydrothiinone 9a undergoes photocycloaddition regioselectively to all three C=C bonds of penta-1,2,4-triene (10), the relative stabilities of the biradical intermediates determining the product distribution. In contrast, cyclohexenone 9b and dihydropyranone 9c afford more complex mixtures of bicyclo[4.2.0]octanones, which also turn out to be less stable on chromatographic workup, reflecting the higher strain due to the shorter bond lengths (CÀO and CÀC vs. CÀS) in the six-membered rings, respectively.1. Introduction. -Bicyclo[4.2.0]octan-2-ones 1, resulting from photocycloaddition of cyclohex-2-enones 2 to alkenes, represent important synthons, as they are easily transformed into a plethora of natural and nonnatural target molecules [1]. As these light induced conversions proceed in a stepwise manner via 1,4-biradicals as 3, they tend to afford diastereoisomeric mixtures as 1a and 1b, when the alkene 4 is unsymmetrically substituted. Recently, we extended the scope of such photoannelations by obtaining naphthalenones 5 in reactions of 2 with conjugated enynes, e.g., 2-methylbut-1-en-3-yne (6), as (photo-inert) alkene partner [2] [3]. Bicyclic ketones 5 result from 1,6-cyclization of alkyl propargyl biradical 7, followed by a 1,3-H-shift in the cyclohexa-1,2-diene 8 (Scheme 1). In pursuing related examples wherein 1,4-and 1,6-cyclization of the biradical intermediate could compete, we investigated the reaction of cyclohex-2-enones 9a -9c in the presence of the conjugated en-allene penta-1,2,4-triene (10).

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

“…Extensive studies by the Margaretha group aimed to elucidate the intermolecular [2 + 2] photocycloaddition chemistry of 2,3-dihydro-4 H -pyran-4-ones ,− and 2,3-dihydro-4 H -thiopyran-4-ones. − An immense data set has been generated, out of which two examples are given in Scheme . Dihydropyranone 256 produced with high regioselectivity the respective HT photocycloaddition product.…”

Progress in the Synthesis and Transformations of Alkylidenecyclopropanes and Alkylidenecyclobutanes

“…The photocycloaddition of cyclohex-2-enones to penta-1,2,4-triene (vinylallene) affords complex mixtures of regioisomeric and stereoisomeric products in which all three CC bonds of the penta-1,2,4-triene were involved . The thermal dimerization of allenes gives dienes of type 1051 in most cases .…”

Reagent‐Controlled Regioselective Intramolecular [2+2] Cycloaddition between Vinylsilanes and Allenes

“…[1,2] The [2 + 2] cycloaddition reaction of allenes has traditionally been used in synthesizing cyclobutanes, which have a high ring strain. [3,4] Previous reports can be divided into three major methodologies (Scheme 1): photochemical [2 + 2] cycloadditions, [5] thermal cycloadditions, [6] and transition-metalcatalysed cycloadditions. [7] However, there are few examples of proximal site-selective reactions, as most reactions occur at less bulky distal sites or have poor regioselectivity.…”