Photochemische Bildung von Heteromethylencyclopropanen, 20. Diastereomere 3,3,5,5-Tetraalkyl-3,5-dihydro-4H-pyrazol-4-one

✍ Scribed by Quast, Helmut ;Jakobi, Harald ;Seiferling, Bernhard

• Book ID: 102902280
• Publisher: John Wiley and Sons
• Year: 1991
• Tongue: English
• Weight: 608 KB
• Volume: 1991
• Category: Article
• ISSN: 0947-3440
• DOI: 10.1002/jlac.199119910107

✦ Synopsis

Photochemical Formation of Heteromethylenecyclopropanes, 20^1)^). – Diastereomeric 3,3,5,5‐Tetraalkyl‐3,5‐dihydro‐4H‐pyrazol‐4‐ones

The dibromo ketones (u/1)‐8b react with sodium azide to afford the α,α′‐diazido ketones (u/1)‐9b, which are reduced to the diastereomeric α,α′‐diamino ketones (u/1)‐11b. The hydrochlorides of the latter are separated through fractionating crystallization yielding pure (1)‐11b · 2 HCl (d.e. >99%) and (u)‐11b · 2 HCl having d.e. of 60%. Oxidation of (u)‐11b and (1)‐11b with sodium hypobromite produces the 4‐pyrazolones cis‐6b (d.e. = 60%) and trans‐6b (d.e. >99%), respectively. The mixture of cis‐and trans‐6b is reduced with sodium tetrahydridoborate to the diastereomeric alcohols cis,cis‐, cis,trans‐, and trans,trans‐5, which exhibit separation factors of α = 1.08 and α = 1.15, respectively, in the HPLC on silica gel. The diastereomeric alcohols are separated on large scale (d.e.'s >99%) by means of cyclic medium‐pressure chromatography (silica gel/ petroleum ether/ethyl acetate). Swern oxidation of cis,cis‐5 and trans,trans‐5 produces high yields of cis‐6b, while cis,trans‐5 affords trans‐6b. Both ketones possess d.e.'s >99%.