106(P-63) 触媒的不斉ヘテロDiels-Alder反応を鍵反応とするCryptomoscatone E2の合成研究(ポスター発表の部)

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Cryptomoscatone E2 (1) isolated from Cryptocarya moscata, Lauraceae, is biosynthesized from mixed acetate-shikimate pathway and structurally characterized by 5,6-dihydro-α-pyrone, 1,3-diol and trans-styryl skeletons. Although the absolute configuration at C6 was reported to be R based on its CD spectrum and the relative stereochemistry of the 1,3-diol to be anti by ^1H NMR analysis, its complete stereochemistry remained unestablished. In order to determine the absolute configuration of 1, we addressed to the synthesis of its two possible diastereomers, (6R, 10S, 12R)- (1_R) and (6R, 10R, 12S)-isomers [in its enantiomeric form, (6S, 10S, 12R)-isomer (1_S)]. We recently developed asymmetric hetero Diels-Alder (AHDA) reaction of Danishefsky diene with aldehydes catalyzed by chiral (salen)chromium complex 2, which proceeds with predictable enantioface selectivity (Scheme 2). Using the AHDA reaction as a key reaction, the two diastereomers of 1, 1_R and 1_S, were synthesized in an enantioselective manner. The synthesis started with AHDA reaction of trans-cinnamaldehyde3 and Danishefsky's diene 4 giving γ-pyrone (R)-5 (95% ee, quantitative). Reduction of 5 with LiAlH_4 gave 6 as a single isomer (Scheme 3). After hydrolysis of 6, the resulting hemiacetal 7 was two-carbon extended by Wittig-Horner reaction to give diol 7. Diol 7 was protected as TBS ether and transformed to aldehyde 9 (Scheme 4). AHDA reaction of 9 and 4 utilizing (R,R)-2 and (S,S)-2 as the catalyst gave γ-pyrone 10_R and 10_S, respectively. γ-Pyrones 10_R and 10_S were converted into α-pyrones (11_R and 11_S), respectively, by the sequence: i) reduction under Luche's conditions, ii) acidic transformation of enol to methyl acetal, and iii) Jones oxidation. Finally, deprotection of TBS ether gave 1_R and 1_S. (Scheme 5). Spectroscopic (^1H NMR, ^<13>C NMR, CD) data of ^1R and 1_S. were collected. CD spectra of these isomers were found to be almost mirror images from each other and 1_R showed positive sign in a range of 250-270 nm as reported for 1. Both ^<13>C NMR data of 1_R and 1_S were in good agreement with the reported one of 1. ^1H NMR spectra of 1_R and 1_S were also very similar to each other but different in the splitting patterns of the C11 methylene protons. ^1H NMR spectrum of 1 is in request.
天然有機化合物討論会の論文
2001-09-01

106(P-63) 触媒的不斉ヘテロDiels-Alder反応を鍵反応とするCryptomoscatone E2の合成研究(ポスター発表の部)

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