Ketones, bridged polycyclic

These transformations were applied to develop a new promising method for synthesis of various bridged polycyclic systems66, viz. ketones 160 and 161. Tropone reacts with butenyl magnesium bromide (—78 °C, 75%) to form a mixture of 2-(3-butenyl)dihydrotropones 158 and 159, the pyrolysis of which (200-210 °C, neat or in heptane solution) leads to 60% total yield of the isomeric homoprotoadamantenones 160 and 161 and the tricyclic ketone 162 in a ratio of 58 18 24, respectively (equation 49)66. 

A reaction between a-diazo ketones and a,(1-unsaturated aldehydes under rhodium(ll)-catalysis provides a route to epoxy-bridged tetrahydropyran-4-ones <2002JOC8019>. This methodology allows entry to functionalized spiro-dioxa-bridged polycyclic frameworks <2002TL3931, 2003T8117>. 

In many cases, bridged polycyclic ketones react relatively slowly in the Meerwein-Ponndorf-Verley reduction and the thermodynamic equilibrium is obtained using aluminum isopropoxide in boiling isopropanol. The equilibrium depends on the substituents (see refs 5, 183 and 184). 

Synthetic applications of [4+2] photocycloadditions have not been extensively developed. It has been observed, however, that intramolecular photocycloaddition of arenes and allenes proceeds preferentially via the [4+2] reaction mode. This process appears to be reasonably general across a diverse range of allenes attached to aromatic aldehydes and ketones with a variety of tethers. Thns, the protected aniline 36 undergoes para-cycloaddition to give the intriguing bridged polycyclic product 37 (Scheme 15.13) [34]. Mechanistic details of this and related transformations have not yet been reported. 

Sugiyama, N., Nishio, T, Yamada, K., and Aoyama, H., Photochemical reactions of bridged polycyclic ketones. Bull. Chem. Soc. Jpn., 43, 1879,1970. 

The procedure described here is a modification of one involving the thermal fragmentation of 1-adamantyl hypoiodite and cycliza-tion of the resulting iodo ketone/ By means of this procedure, 4-protoadamantanone is obtained from 1-adamantanol with consistent yields in the range of 71 to 82% and a purity greater than 98%. This method is also applicable to the preparation of other polycyclic ketones from the related bridgehead alcohols with a-bridges of zero, one, or two carbon atoms (see Table I). 

These examples show that the control of dichlorocarbene addition is hard to predict in the case of enones or dienones. The following example demonstrates that the regioselectivity of the carbene cycloaddition significantly increases if the starting polycyclic ketone is bridged by the ethylene rather than methylene chain, e.g. formation of 13, 14 and 15. ... 

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