Steric compression enones

Enone Photorearrangements. As will be seen, the steric compression is associated with the substituents attached to the... 

The results confirmed our working hypothesis. Pyramidalization at Ca of enone and Co of led to no significant steric compression (contacts >2.2 A), whereas pyramidalization in all other cases led to new contacts averaging (at their minimum)... 

A. These results are summarized in Table I. As an example, Figure 4 shows stereodiagrams of enone g, before and after pyramidalization at Cq. The steric compression accompanying full 55° pyramidalization is indicated by the dotted lines and consists of hydrogen-hydrogen contacts of 1.71 and 1.87 A pyramidalization at Ca is unimpeded. 

Figure 4. Stereodiagrams of enone lg, before (above) and after (below) pyramidalization at the -carbon atom. The steric compression contacts which develop are shown by the dotted lines.

Reactions of Enols and Enolate Anions.—Several methods are described for transposition of an oxo-function to the adjacent site. They involve formation of a suitable a-substituted derivative (hydroxymethylene ° or benzylidene ) and subsequent steps which transform the substituent into an isolated oxo-group. Condensations leading to both the 2-hydroxymethylene- and the 2-arylidene-3-oxo-steroids are described for 3-ketones of the 5jS-series, and also of the 5j8,9j5,10a-( retro ) series.Condensations of aromatic aldehydes at C-2 in the 5 -series are unusually slow enolisation towards C-4 is preferred, but steric compression between C-4 and C-6 in 5/3-compounds severely hinders the condensation reaction at C-4, allowing reaction at C-2 via the 2-enol. Reduction of a 21-hydroxymethylene-pregnan-20-one (337) with sodium borohydride afforded the homopregnanediol (338), although reduction of enolised P-dicarbonyl compounds frequently proceeds via elimination to give enones, and thence allylic alcohols. 

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