Benzonorbomadiene derivatives

A final comment on Table 4 concerns the reaction shown in entry 8. Because the di-TT-methane photorearrangement of benzonorbomadiene derivatives requires triplet energy sensitization, we could not use typical, passive amines such as (/ )-( + )-l-phenylethylamine as chiral auxiliaries. We therefore prepared an optically pure amine to which a sensitizing benzophenone moiety was tethered, namely, the 4-benzoylphenyl ester of l-valine [25]. Photolysis of the salt of this amine at wavelengths where only the benzophenone chromophore absorbs led to the photoproduct in 91% ee at 100% conversion, a gratifying vindication of the concept. Optically active photosensitizers have been used in solution with limited success [33], but this represents the first example of simultaneous triplet-triplet energy transfer and asymmetric induction in the crystalline state. 

The JV-benzyl aziridines 69, 92-95 which differ only in the nature of the norbomane bridge (CH2, spirocyclopropyl, isopropylidene, oxygen, substituted nitrogen) (Scheme 14), have been prepared and reacted with each of the corresponding benzonorbomadienes 36-40 from which they were derived. All 25 reactions were conducted to produce 14 of the possible 15 different... 

Salt crystals of the benzonorbomadiene carboxylic acid 68 with optically pure triplet sensitizer amines 70, 71 also underwent enantioselective di-TT-meth-ane photorearrangement to afford the optically active tetracyclo[5.4.0.02,4.03 6]undecane derivative 69 in high enantiomeric excess [81]. The 4-acetylbenzyl ester of L-phenylalanine 70 and the 4-benzoylphenyl ester of l-valine 71 perform the dual roles of asymmetric induction and triplet energy sensitization. 

Grignard-based benzyne reactions typically produce numerous benzyne-derived by-products. For this reason, yields of benzonorbomadiene via this route are often moderate (-50%), as we also observed on large-scale. Fortunately, the by-products are higher in molecular weight, allowing purification based on boiling point. Additional removal of undesired products from the distilled cmde reaction extracts (typically >90% pure) was achieved in the dihydroxylation step with OSO4, which sequestered all benzonorbomadiene as crystalline analytically pure diol 4 (see Scheme 3.1 and Scheme 3.4). 

An enzymatic approach has been advocated for the single-step preparation of small quantities of enantiomerically enriched 2-oxynorbomanes and 2-oxybicyclo[3,2,l]-octanes. Thus, for example, horse fiver alcohol dehydrogenase-catalysed oxido-reduction of ( )-2-norbornanone results in the production of (—)-(lR,4S)-2-nor-bornanone and (-f)-(lS,21, 4R)-endo-2-norbomanol of up to 46 and 66% optical purity respectively. The space-conjugated olefinic double bonds in various nor-bornene and bicyclo[2,2,2]octene derivatives are rapidly reduced by lithium in liquid ammonia benzonorbomadiene, for example, is reduced to benzonorbomane. ... 

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