Unsymmetric bridgehead

Contrary to the rotationally symmetric bridgehead substitution (azoalkanes 5a,b,c), which leads to the unselective triplet reaction channel and selective formation of anti-configured (retained) housanes (Scheme 7), the direct photolysis of the rotationally unsymmetric bridgehead-substituted azoalkanes 5d,e is moderately diastereoselective for both the singlet and triplet modes of photolysis (Scheme 9), as evidenced by the synjanti (inversion/retention) ratio. Thus, the photolysis of 5d,e affords under singlet conditions (high-temperature direct photolysis) predominantly the retained housanes anti-14d,e syn anti 21 79 for housane 14d and 33 67 for 14e) while under triplet conditions (low-temperature direct or benzophenone-sensitized photolysis), the inverted diastereomer syn-14d,e is favored syn anti 61 39 for 14d and 70 30 14e). 

It was found that [2- " C]ornithine labelled only one of the bridgehead positions. It follows from this that the incorporation of the amino acid is unsymmetrical and cannot therefore involve the symmetrical base putrescine as an intermediate. As a result of corroborating tracer experiments obtained for tropane alkaloids and cuscohygrine 6.68), it is currently believed [44] that unsymmetrical ornithine incorporation is achieved through methylation to AT-methylornithine 6.61) followed by decarboxylation to give A-methylputrescine 6.62) this latter unsymmetrical base may also be formed from putrescine 6.63). It is to be noted that this pathway differs from that deduced for piperidine alkaloids, where non-symmetrical incorporation of the precursor amino acid (lysine) is more simply accounted for (Section 6.2.1). 

This unprecedented inversion for the triplet pathway is rationalized in terms of the unsymmetric nature of the n-propyl and acetoxymethyl substituents in regard to rotation about the bridgehead position of the planar cyclopentane-1,3-diyl triplet biradicals DR(d,e) cf. Scheme 9. For the lower-energy conformation of the DR(d,e) biradical, the X fragments of the bridgehead substituents points, for steric reasons, away from the annellated cyclopentene ring and are preferably located on the upper side of the... 

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