Chiral allenes, photocycloaddition

Vinylogous thioesters (A3, Q = S) have been used less frequently in [2 + 2]-photocycloaddition as compared to their oxygen and nitrogen analogues [83], More recent applications can be found in the above-mentioned study with chiral allenes [79]. 

When the stereogenic center is a chiral allene, intramolecular 12 + 2 photocycloaddition produces optically active adducts [117]. A recent reinvestigation of the photoreactivity of optically active allenic enones (156) has shown that the adducts 157a and 158a are obtained with 100% asymmetric induction [146] (Scheme 24). 

Scheme 24. Intramolecular [2 + 2] photocycloaddition Asymmetric induction when the stereogenic center is a chiral allene.

A very useful extension of the de Mayo reaction has been recently introduced by Blechert et al. (Scheme 6.26) [78]. The retro-aldol fragmentation was combined with an intramolecular enantioselective allylation (asymmetric ring-expanding allylation) catalyzed by a chiral Pd complex. Bicycloheptane 68, for example, was accessible by intermolecular [2 + 2]-photocycloaddition of cyclopentenone 67 with allene. Further transformation in the presence of Pd2(dba)3 (dba = dibenzylideneacetone) and the chiral oxazoline ligand 69 (tBu-phox) resulted in the enantioselective formation of cycloheptadione 70. 

Substrates A3 (Q = O) have been employed not only as starting materials for fragmentation reactions but also to probe novel stereoselectivity concepts. The photochemical transformation of axial chirality into central chirality was achieved by Carreira et al., who employed chiral, enantiomerically pure allenes in intramolecular [2 + 2]-photocycloaddition reactions (Scheme 6.27) [79]. The reaction of enantiomerically pure (99% ee) cyclohexenone 71, for example, yielded the two diastereomeric products 72a and 72b, which differed only in the double bond configuration. Apparently, the chiral control element directs the attack at the allene to its re face. The double bond isomerization is due to the known configurational liability of the vinyl radical formed as intermediate after the first C—Cbond formation step (see Scheme 6.2, intermediate C). 

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