Cyclohexenone, 2-methyl cycloaddition reactions

Asymmetric induction has been observed in some of these cycloaddition reactions with the use of chiral enamines52-54. For example, reaction of L-proline derived enamines (e.g. 95) with methyl vinyl ketone (equation 17) afforded, after hydrolysis, chiral cyclohexenones (e.g. 96) with optical yields up to 50%52. 

The stabilizing effect of the secondary orbital interactions of the trifluoromethyl substituent with dienes is exemplified in the cycloaddition reaction of a-(trifluorornethyl)styrene (7) with Danishefsky s diene 4. A phenyl substituent is known to stabilize the endo-phenyl transition state. Furthermore, the steric hindrance of a trifluoromethyl group should favor its exo approach. However, cycloadducts 8 are obtained in a 1 1 mixture of transicis stereoisomers which are converted into the cyclohexenone 9. Under the same thermal conditions, a-methyl-styrene is unreactive.5... 

The cyclic a,/ -unsattirated ketone cyclohex-2-en-l-one (50) was used as building block in the one-pot domino cycloaddition of enol ether 14 and nitrostyrene 15a. At 15 kbar and 50 "C, nitroso acetal 51 was formed in 67 % yield, whereas nitroso acetal 54a was formed as a side product (Scheme 9.18). This result indicated that the 1,3-dipolar cycloaddition is still faster with the electron-poor substituted cyclohexenone 50 than with the electron-rich mono-substituted enol ether 14. The one-pot reaction of 52 with enol ether 14 and nitrostyrene 15a merely resulted in formation of nitroso acetal 54a instead of nitroso acetal 53. The unwanted side reaction was not observed in the one-pot three-component reaction with 14 and methyl-substituted nitrostyrene 15b and 52 (Scheme 9.19). The large difference in reactivity between the three components in both the Diels-Alder and the [3 + 2] cycloaddition resulted in the formation of 55 as the main product. The side reaction of 16b with 14 to form 54b was prevented, since 14 was completely consumed in the reaction with 15b to give nitronate 16b (15 kbar, 50 °C, 16 h). However, heating (50 °C) the reaction mixture for 76 h at 15 kbar was necessary to produce nitroso acetal 55, which was formed as a mixture of two major diastereomers (ratio 3 1) in 69 % yield. 

Investigation of [2 -l- 2] photocycloaddition of enones to monosubstituted alkynes indicates that the reaction proceeds with an opposite regioselectivity compared to that of alkenes [71]. Generally, head-to-head adducts are major products except when the group substituted on the alkyne is -CO2R or -OR (see Scheme 17). Thus, methyl propynoate (60, R = CO2CH3) photoadds to cyclopentenone 59 to give 1 1 HH and HT products. Cycloaddition of cyclohexenone to 1-hexyne has also been studied and the HH HT ratio varies from 2 1 to 7 1 as a function of substituents at the enone 3-position. It... 

A detailed study of the cycloaddition of trifluoromethylethene has appeared as has work on a-siloxy-a -unsaturated ketones as dienophiles under Lewis-acid catalysis. Amongst the new dienophiles that have been reported are the two novel ketene equivalents methyl methoxypropiolate (129) and methyl phenyl-thiopropiolate (130). A series of 3-(acyloxy)but-3-en-2-ones (131) has been prepared and used as dienophiles in a route to anthracylinones, and, as part of a route to highly functionalized cyclohexenones, the vinyl sulphone (132) has been used in a Diels-Alder reaction with various dienes. ... 

---