Alkanones reaction with alkenes

Aldol reaction between a ketone and an aldehyde can be controlled by prior conversion of the ketone into the ethyloxalyl derivative before reaction with the aldehyde. The keto-lactone so formed is decomposed to an alkene moiety with base. This method is particularly well suited to the formation of a-alkylidene-cyclo-alkanones (ring size >6) and a-alkylidene-y-lactones (Scheme 73). ... 

In the previous sections the multiplicity of the excited state has been discussed. In general, electron-deficient alkenes react preferentially with alkanones in the nit singlet state preserving the stereochemistry of the alkene (see Section I.6.I.4.3.2.)6,, 3 27,47-49. Note that ZjE isomerization of the alkenes can precede and as a consequence change the stereoselectivity of oxetane formation61 J 27. Nevertheless, the majority of Paterno- Biichi reactions are concerned with electron-rich olefins and here triplet states of the carbonyl compounds are most likely involved. As briefly discussed in Section 1.6.1.4.3.1., 1,4-diradicals play a decisive mechanistic role and consequently also control the stereochemical outcome some examples have already been given11 28 29 32-J6. The controlling factors can be seen in the mechanistic scheme shown in... 

In Fig. 4, reaction A is a highly stereoselective reduction of 1-aryl alkanones with (-)-chlo-ro diisopinocampheylborane [21]. Upon co-ordination of the ketone oxygen with the Lewis acidic chirotopic and non-stereogenic [22] boron atom of the chiral reagent, two diastereo-isomeric complexes arise. The sterically less hindered one is preferentially formed and leads the major (,S)-enantiomer, which is isolated after a work-up that allows recovery of a-pinene, the chiral alkene from which the borane is prepared. 

Quantum yields of acetone and the corresponding alkene product of the Norrish type-II reaction for the 2-alkanones were found to be near equal in preliminary experiments of Golemba and Guillet (1972). In most experiments, they used the alkene to monitor the extent of the type-11 reaction. The results showed a small decrease in (pii with increase in chain length of the ketone, which is shown in parentheses 0II = 0.25(6) 0.20 (7) 0.20 (8) 0.20 (9) 0.20 (11) 0.15 (19). Solution-phase photolyses of the ketones were made with added triplet molecule quencher, cis-1,3-cyclooctadiene, at concentrations sufficient to quench virtually all of the triplet reactions. From such experiments, Golemba and Guillet (1972) found the ratios of the quantum yields of the triplet to that of the singlet participation in reaction (II) (fii/(pii = 0.65 with 2-hexanone 0.61 with 2-octanone 0.64 with 2-nonadecanone. 

---