Ring-opening of cyclohexene oxide

Table 12. Asymmetric ring opening of cyclohexene oxide with anilines catalyzed by complex 73 under microwave irradiation...

Adolfsson, H. Mobrg, C. (1995) Chiral Lewis acid catalyzed asymmetric nucleophilic ring opening of cyclohexene oxide., Tetrahedron Asymmetry, 6 2023-2031. 

Mirkin and coworkers reported on catalytic molecular tweezers used in the asymmetric ring opening of cyclohexene oxide. In this case the early transition metal is the catalyst and rhodium functions as the structural inductor metal. The catalyst consists of two chromium salen complexes, the reaction is known to be bimetallic, and a switchable rhodium complex, using carbon monoxide as the switch. Indeed, when the salens are forced in dose proximity in the absence of CO the rate is twice as high and the effect is reversible [77]. 

Scheme 7. Catalytic enantioselective ring opening of cyclohexene oxide with trimethylsilyl azide using the chiral Cr(lll) - salen complex 1 [26],...

The stereochemistry of the cyclohexene adduct was established as cis by comparison of the product with the authentic tnms-isomcr prepared by ring opening of cyclohexene oxide with tert-butylamine. Similarly, the products obtained from (Z)- and ( )-l-deuterio-l-decene were converted to diastereomeric oxazolidinones which were compared with the authentic diastereo-mers. Furthermore, different diastereomers were obtained from ( )- and (Z)-l-phenylpropene, It is therefore reasonable to assume complete syn addition for all alkenes. 

The first attempt at enantioselective ring opening of meso-epoxides by using a chiral selenolate was reported in 1988 [90]. Enantiomerically pure seleno-binaphthyl compounds 65-67 were synthesized and applied to the asymmetric ring opening of cyclohexene oxide (Scheme 47). 

Since ring closure is only possible when the starting material is diaxially substituted, this has to mean that ring opening is similarly only possible if the product is diaxial. This is a general principle ring opening of cyclohexene oxides always leads directly to diaxial products. The diaxially substituted product may then subsequently flip to the diequatorial one. 

Emziane, M., Sutowardoyo, K. I., Sinou, D. Asymmetric ring-opening of cyclohexene oxide with trimethylsilyl azide in the presence of titanium isopropoxide/chiral ligand. J. Organomet. Chem. 1988, 346, C7-C10. 

Epoxides can serve as competent electrophiles in the alkylation of a variety of carbanions, as illustrated by the ring opening of cyclohexene oxide (64) with the dianion of phenylacetic acid (77) to produce the y-hydroxy carboxylic acid 78. In this protocol, the dianion is generated using K-butyllithium and a substoichiometric quantity of a secondary amine lithium chloride is also used as a Lewis acid additive to activate the secondary epoxide toward nucleophilic addition. Primary epoxides undergo addition without the use of catalyst—in these cases, the nucleophile attacks at the less substituted position <04EJOC2160>. 

A titanium (IV) complex with the identical (salen) figand was reported recently to effect the addition of thiophenol with moderate enantioselectivity [18]. The complex was formed in situ from 5 mol % Ti(0-z-Pr)4 and 5.5 mol % of the chiral ligand 9, and this catalyst promoted the ring opening of cyclohexene oxide in 93% yield and 63% ee at -40 to -25 °C (Scheme 6). 

Jacobsen reported that the Co(salen) complex 10 catalyzed the addition of carboxylic acids to meso-epoxides [20]. An initial screen revealed that benzoic acid and its derivatives were the most useful nucleophiles from the perspective of reactivity and selectivity. Although optical purities exceeding 90% ee were observed only with selected substrates, the crystalHnity of the benzoate esters in some cases allowed enhancement of their enantiopurity by recrystallization. The ring opening of cyclohexene oxide, for instance, proceeded on a multigram scale in quantitative yield and 77% ee subsequent recrystallizations of the monobenzoate ester 11 then afforded 98% ee material isolated in 75% yield (Scheme 7). 

The complexes of polymer 243 (Scheme 97) with YbCU and LuCls have been used for the ring opening of cyclohexene oxide by trimethylsilylcyanide [154]. Although the conversions were excellent, the ee were low to moderate. Furthermore a significant drop of the enantioselectivity was observed when the recovered supported Yb-complex was reused. 

With a little more sophisticated system employing benzoyl fluoride and a homochiral base, the enatioselective, nucleophilic ring opening of cyclohexene oxide was achieved in moderate to good yields (see 648) [251]. 

---