1,5-Hexadiene photocycloaddition reactions

The skeleton of 47 is a heterocyclic tricyclo[6.2.0.0 ]decane and the similarity to the tricyclic kelsoene is obvious. In the course of the above-mentioned studies we had become curious whether the high facial diastereocontrol in the photocycloaddition reaction could be extended to other bridged 1,6-hexadienes. Kelsoene was an ideal test case. The retrosynthetic strategy for kelsoene along an intramolecular [2+2]-photocycloaddition pathway appeared straightforward. To avoid chemoselectivity problems the precursor to kelsoene should not contain additional double bonds. Alcohol 48, the hydroxy group of which was possibly to be protected, seemed to be a suitable substrate for the photocycloaddition (Scheme 14). Access to the 1,2,3-substi-... 

Transient absorption spectra of singlet diradical in addition to exciplex have been observed in the course of the photocycloaddition reaction of 9-cyanoan-thracene (CNA) with 2,5-dimethyl-2,4-hexadiene (DMHD), while no intermediate was detected in CNA-l,3-cyclohexadiene (CHD). The cycloadduct with CHD shows an efficient adiabatic photodissociation into CNA and CHD immediately after excitation. 

The nonstereospecific (using Zimmerman s definition) [108] nature of this reaction has been demonstrate by the use of stereoisomeric substrates which lead to oxetanes 114 in identical diastereomeric ratios. Another possibility for obtaining stereoisomerically pure oxetanes is the use of alkenes which simultaneously serve as quenchers for the carbonyl triplets, e.g., dienes. The photocycloaddition of acetone and 2-methyl-2,4-hexadiene (115) represents such a process leading to two regioisomeric oxetanes 116 and 117 where the substrate configuration is retained in 117 [75]. 

A few specific examples of cyclopropanation using the above methods are shown in Scheme 2.134. The naturally occurring insecticide /ra -chrisanthemic acid 396 served as an obvious target to check the viability of carbene addition as a preparative method. This compound was first synthesized (in the mixture with the cis isomer) by the monocyclopropanation of 2,5-dimethyl-2,4-hexadiene. Since then, numerous analogs of 396 were prepared by similar reactions. Some of the analogs are now widely used as efficient and ecologically safe pesticides. The formation of the tricyclic hydrocarbon 397 from 1,5-hexadiene proceeds as a sequence of inter- and intramolecular carbene transfer reactions. An initial carbene precursor, CHBr3, is actually employed here as an equivalent of a unique tetradentate Ci synthon The preparation of 398 via intramolecular [2 + 1] photocycloaddition is a typical example of the efficiency of this route for the construction of the polycyclic framework frequently encountered in the structures of natural compounds. 

---