- Studies on the synthesis of acanthodoral and nanaimoal: Evaluation of cationic cyclization routes
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Intramolecular Lewis acid-promoted reactions of α,β-unsaturated ketone 6 and aldehydes 7 and 8 were examined as potential routes to acanthodoral (1), a structurally interesting natural product. Ketone 6 afforded ene product 22 exclusively, and both 7 and 8 gave mixtures of bicyclic aldehydes 3 and 26 and tricyclic aldehyde 25. The latter most likely results from 7 by intramolecular cyclization of the alkene onto the Lewis acid-activated carbonyl moiety affording carbocation 31 followed by a 1,2-hydride shift and ring closure. Starting from 8, tricyclic aldehyde 25 apparently forms by cyclization to cation 35 and ring closure to cyclobutane 36, followed by ring opening to 31, the same cation as formed in reactions of 7. Nanaimoal (3) results from loss of H+ from 31, and bicyclic aldehyde 26 may be formed in a similar manner or by a concerted ene reaction. The configuration of 25 establishes that the stereochemistry of the initial cyclization to 31 precludes the possible use of this strategy for the synthesis of acanthodoral. However, acid-promoted cyclization of allylic alcohol 23 efficiently gives diene 29 which undergoes selective hydroboration/oxidation to afford nanaimoal.
- Engler,Ali,Takusagawa
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p. 8456 - 8463
(2007/10/03)
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- Photochemical Reactions; Photochemistry of Acylsilanes: 1. Siloxycarbene Formation versus γ-H-Abstraction
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The syntheses and the photolyses of the acylsilane 1 and the corresponding methyl ketone 2 are described.On n,?*-excitation, the silyl ketone 1 as well as the methyl ketone 2 undergo a Norrish type II reaction involving γ-H-abstraction and fragmentation to the diene 12, and acetone (20) or the acylsilane 26, respectively.The methyl ketone 2, but not the acylsilane 1, isomerizes to cyclobutanols (21A-D).Additionally, compound 1 shows photochemical behavior typical of acylsilanes undergoing rearrangement to the siloxycarbene intermediate c.Insertion of c into the O-H-bond of the enol 28 leads to compound 13.Initial trapping of the siloxycarbene c by H2O, however, gives rise to the formation of compounds 16-18.As minor photolysis products of 1, the isomers 14 and (Z)-15 were formed; however, on vapor phase thermolysis (520o) of 1, compounds 14 and (E/Z)-15 were obtained in 92percent combined yield.To a small extent the acylsilane 1 also undergoes Norrish type I cleavage leading to the acid 19.
- Scheller, Markus E.,Frei, Bruno
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p. 1734 - 1747
(2007/10/02)
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