| dc.description.abstract | The synthesis of the ‘‘functionalized’’ Hagemann’s ester (S)-
18 was investigated. The common starting material in these
approaches was enamino ester (S,Z)-5, which was prepared
through the condensation of keto diester 4 with (S)-1-phenylethylamine.
The Michael addition reaction of 5 with methyl
vinyl ketone gave the expected adduct (S)-6 with an ee $
95%. However, all attempts at annulation of 6 invariably afforded
the unwanted cyclohexenone derivatives 7 or 8. The
addition of 5 to Nazarov reagent 9 furnished adduct (S)-10
with an ee $ 95%. The Triton B-induced annulation of 10
unexpectedly gave aldol 11. Depending on the reaction con-ditions, annulation of 11 afforded either the bicyclic lactone
12, or cyclohexenones 13 or 15. An efficient way of reversing
the sense of the regiochemistry of the previous annulation
was found, based on the use of diethyl 2-oxo-3-vinylphosphonate
(16) as a Michael acceptor. Thus, the condensation
of 5 with 16 gave (S)-17 with an ee $ 95%, and cyclization
of (S)-17 under Horner−Wadsworth−Emmons conditions
gave the desired Hagemann’s ester (S)-18. The structural assignments
for 18 were ascertained by chemical correlation
with the known hydrindenedione (S)-21. | en_US |
