Cis-3-hexenoic acid

An example of a stereoselective hydrogenation in ionic liquids was recently successfully demonstrated by Drie en-H6lscher et al. On the basis of investigations into the biphasic water/n-heptane system [51], the ruthenium-catalyzed hydrogenation of sorbic acid to cis-3-hexenoic acid in the [BMIM][PFg]/MTBE system was studied [52], as shown in Scheme 5.2-8. 

The hydrogenation of sorbic acid to cis- and trans-3-hexenoic acid using the ionic ruthenium complex [Ru(C5Me5)(MeCH=CH-CH=CHC02H)][CF3S03] as the catalyst has been conducted in [BMIM][PFg]-methyl tert-butyl ether (MTBE) [40]. The formation of ds-3-hexenoic acid can be achieved with selectivity up to 93% [Eq. (4)]. 

Examples of 1,3-asymmetric induction in cyclizations to 8-lactones have been observed. Iodolactonization of 3-methyl-5-hexenoic acid to a 8-lactone under equilibrating conditions showed reasonable stereoselectivity (6 1 cis trans).l20b Recent studies have examined the formation of 8-lactones from cyclization of 5-hexenoic acids with a homoallylic oxygen substituent at C-3.135 Selenolactonization of 3-hydroxy-5-hexenoic acid under conditions of kinetic control provided the trans lactone in modest yield (40%) and high stereoselectivity.13515 Equilibrating conditions led to a slight preponderance of the cis... 

By treatment of ( )- and (Z)-2-methoxy- and 2-acetoxy-3-methyl-4-hexenoic acids (11) with iodine in acetonitrile, the 3-methoxy- and 3-acetoxydihydro-5-(1-iodoethyl)-4-methyl-2(3//)-furanones (12) have been synthesized in good yield and d.r. (trans/cis) 95 518a. 

To a mixture of 0.75 g (9 mmol) of NaHC03 and 39 mg (0.30 mmol) of4-methyl-5-hexenoic acid (l)in 1 raL of CHjCN at 0 "C is added 0.23 g (0.91 mmol) of iodine. After 3 h the mixture is diluted with Et20, washed with sodium thiosulfate and water and concentrated yield 64mg (83%) d.r. (cis/irons) 70 30. 

When 3-methyl-5-hexenoic acid (1) is cyclized under either kinetic or thermodynamic conditions, preferential formation of the cis-lactone is observed, although a lower selectivity is obtained under kinetic control1. 

Low asymmetric induction is observed when (R)-3-benzyloxycarbonylamino-5-hexenoic acid (1, R = H) is iodolactonized under kinetic conditions (iodine/potassium iodide/sodium hydrogen carbonate). A mixture of the cis- and trww-iodo-5-lactones (60 40) in 94% yield is obtained. Better stereoselectivity can be obtained when the. V-benzyl derivative 1 (R = Bn) is cyclized under the same conditions. In fact, an 86 14 cisjtram) ratio is observed, showing that halolactonization is kinetically favored over halocarbamation, with the bulky substituent at nitrogen inducing high selectivity7. n n... 

When 3-hydroxy-5-hexenoic acid (9) is subjected to selenolactonization under kinetic conditions at —70 "C, a mixture of selenolactones is obtained in 42 % yield and about 91 9 (trans/cis) diastereomeric ratio. 

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