PHOTOPROTONATION CYCLOALKENES

PHOTOPROTONATION OF CYCLOALKENES LIMONENE TO p-MENTH-8-EN-l-YL METHYL ETHER... 

The photoprotonation of cycloalkenes, described in this procedure, is believed to proceed via initial light-induced cis —> trans isomerization of the alkene.4 The resulting highly strained trans isomer undergoes facile protonation. This procedure permits the protonation of cyclohexenes and cycloheptenes under neutral or mildly acidic conditions.5 Since the process is irreversible, high levels of conversion to addition products can be achieved. 

A practical synthesis of 1,3-OX AZEPINES VIA PHOTOISOMERIZATION OF HETERO AROMATIC V-OXIDES is illustrated for 3,1-BENZOXAZEPINE. A hydroboration procedure for the synthesis of PERHYDRO-9b-BORAPHENALENE AND PERHYDRO-9b-PHEN-ALENOL illustrates beautifully the power of this methodology in the construction of polycyclic substances. The conversion of LIMONENE TO p-MENTH-8-EN-YL METHYL ETHER demonstrates a regio-and chemoselective method for the PHOTOPROTONATION OF CYCLOALKENES. An efficient method for the conversion of a ketone to an olefin involves REDUCTIVE CLEAVAGE OF VINYL PHOSPHATES. A mild method for the conversion of a ketone into the corresponding trimethylsiloxy enol ether using trimethylsilyl acetate is shownforthe synthesis of (Z)-3-TRIMETHYLSILOXY-2-PENTENE. 

There are several synthetically useful applications of the photoprotonation of cycloalkenes ... 

These are just a few of the ways that a creative chemist can make use of the photoprotonation of cycloalkenes to induce selective reactivity in synthesis. 

A number of reviews have been devoted to the photoisomerism of alkenes, with the photochemistry of cycloalkenes treated as a subcategory.The geometrical photoisomerization of alkenes was reviewed also in the previous edition." However, the photobehavior of cycloalkenes is not a simple extension of that of acyclic alkenes, particularly in the cases of small- and medium-sized cycloalkenes. Medium-sized (E)-cycloalkenes produced photochemicaUy are highly constrained and rapidly react with alcohols in the presence, or even in the absence, of dilute acid. An account of the photoprotonation-driven cationic reactions, such as addition, isomerization, rearrangement, and fragmentation of cyclohexenes and cyclo-heptenes, in protic solvents has been published. The protonation of strained (E)-cycloalkenes was also reviewed in the previous edition. ... 

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