An intramolecular Diels-Alder approach to the cis ring fused isomer of the 25-hydroxy vitamin D2 Grundmann ketone
摘要:
Stereospecific Claisen and intramolecular Diels-Alder reactions of chiral ester synthon 4 results in conversion of a single asymmetric center of commercially available ester 4 to a vitamin D synthon, the C/D cis Grundmann ketone 3. The addition of propenyllithium to aldehyde 9 was dominated by a chelated anti-Cram transition state and yielded the threo isomer 12a as the major product. The stereochemistry of 12a was determined by X-ray crystallography. Conversion of benzyl-protected erythro isomer 11b to its dimethylacryloyl ester followed by ester enolate Claisen rearrangement led to the C17 and C20 stereochemistry of vitamin D. Addition of a pentadienyl anion to aldehyde 15 gave a tetraene, 17, which underwent an intramolecular Diels-Alder reaction to produce compound 18. Removal of the C16 hydroxyl and hydrolysis gave only the cis-fused isomer of 3.
An intramolecular Diels-Alder approach to the cis ring fused isomer of the 25-hydroxy vitamin D2 Grundmann ketone
作者:Stephen R. Wilson、Linda Jacob
DOI:10.1021/jo00042a014
日期:1992.7
Stereospecific Claisen and intramolecular Diels-Alder reactions of chiral ester synthon 4 results in conversion of a single asymmetric center of commercially available ester 4 to a vitamin D synthon, the C/D cis Grundmann ketone 3. The addition of propenyllithium to aldehyde 9 was dominated by a chelated anti-Cram transition state and yielded the threo isomer 12a as the major product. The stereochemistry of 12a was determined by X-ray crystallography. Conversion of benzyl-protected erythro isomer 11b to its dimethylacryloyl ester followed by ester enolate Claisen rearrangement led to the C17 and C20 stereochemistry of vitamin D. Addition of a pentadienyl anion to aldehyde 15 gave a tetraene, 17, which underwent an intramolecular Diels-Alder reaction to produce compound 18. Removal of the C16 hydroxyl and hydrolysis gave only the cis-fused isomer of 3.