Vinylogous stereoisomer

The latter example (reaction 36) already indicates that the Yang cydization can also be used to synthesize four-membered heterocycles. After light absorption, the a,(3-unsaturated carbonyl compound 84 undergoes intramolecular hydrogen abstraction at the a-position of the carbonyl moiety (reaction 37), leading to the 1,4-biradical intermediate XXX [87]. A radical combination then efficiently yields the spirocyclic P-lactam derivative 85, and only one stereoisomer is formed in this case. In this transformation, the a,P-unsaturated carbonyl function can be considered as being vinylogous to a simple ketone. 

Finally, the type IV chiral building block 15 was synthesized via the iminium reduction of the corresponding iminiun salt derived from the vinylogous urethane 14 as shown in Scheme 5. With all four stereoisomers of the chiral building blocks (I IV) for synthesis of 3-piperidinol alkaloid in hand, we examined the synthesis of prosafrinine,... 

When a vinylogous aldol reaction occurs, an a,P-unsaturated carbonyl compound is generated, which may react in a subsequent intramolecular Michael addition. In 2005, the group of Erase [20] introduced a domino reaction of several salicylaldehy-des and senecioaldehydes to produce tricyclic hemiacetals 39 (Scheme 8.12). Only one stereoisomer was formed in the presence of sodium carbonate with various salicylaldehydes. y-Deprotonation of senecioaldehyde followed by a vinylogous aldol... 

Condensation of the vinylogous carbamate (40) with acetaldehyde and isocyanic acid afforded the pyrimidine derivative (41) which was converted to the urea derivative (42) by Curtius rearrangement in 65% overall yield. Treatment of (42) with acetic acid at 50° afforded exclusively the desired product (39), whereas treatment with trifluoroacetic acid gave a mixture of (39) and stereoisomer (43). The stereochemistry was established by comparing the coupling constants involving H-5 and H-6 with those of saxitoxin (1). Different cyclization mechanisms were suggested for the acetic acid and trifluoroacetic acid catalyzed cyclization to account for the different stereochemistry of the products. 

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