Geraniol cyclization

Exercise 30-5 a. Nerol and geraniol cyclize under the influence of acid to yield a-terpineol. How could the relative ease of cyclization of these alcohols, coupled with other reactions, be used to establish the configurations atthe double bond of geraniol, nerol, geranial, and neral Write a mechanism for the cyclizations. 

Scheme 33 Mechanistic studies on the PET-sensitized cyclization of geraniol.

R = vinyl or ethynyl) predominates (c/. Vol. 1, p. 36 Vol. 2, p. 35). Base treatment of a 2-alkoxypyridinium tosylate of nerol gives expected e.g. limonene 82%) cyclic hydrocarbons whereas the corresponding geraniol salt yields similar amounts of cyclic and acyclic hydrocarbons. SnCU-catalysed cyclization of the N-benzylimine derived from R-(+)-citronellal yields the expected menthylamines after catalytic hydrogenation. ... 

Nerol undergoes the same reactions as geraniol, but cyclizes more readily in the presence of acids. 

Silver(I) catalyzed cyclizations of allenic alcohols (202) lead to 2,5-dihydrofurans (203) (79S743), whilst another mild method for the synthesis of tetrahydrofurans is the intramolecular oxymercuration-demercuration process. Geraniol, when treated with mer-cury(II) acetate and subsequently with sodium borohydride, gave a tetrahydrofuran. 

In a subsequent study, however, Whittaker and co-workers803 showed that the product formed from geraniol, in fact, is the isomeric bicychc ether 198. It was also shown that cyclization of citronellal in HSO3F— SO2 at 78°C affords pulegol and neoisopulegol that is, the transformation parallels closely the reaction in normal acids.804... 

Figure 1.14 a Equation for the cyclization of geraniol, and b possible mechanism in the presence of zeolite Y, a solid acid catalyst that has both Bronsted and Lewis acid sites. 

The same group has further shown that the alkenyl double bond geometry of geraniol- as well as nerol-derived ketones 41 and 43 remains unaffected during the photocyclizations. This suggests that the rate of cyclization is much higher than the rotation about the double bond in the allylic radical center of the biradical (Scheme 8.12). 

Weng, H., Scarlata, C., and Roth, H.D. (1996) Electron transfer photochemistry of geraniol and (E, E)-farnesol. A novel tandem , 1,5-cyclization, intramolecular capture. Journal of the American Chemical Society, 118, 10947-10953. 

This reaction was used for a synthesis of the seven-membered cyclic terpene karahan-aenol (3) from the methyl ether (2) of geraniol by an electrophilic cyclization of an allylsilane.3... 

When geraniol and phenol react together in the presence of 85 % phosphoric acid, there are formed, in addition to o- and p-geranylphenols, the singly cyclized chroman (28) and the doubly cyclized hexahydroxanthene (29). °... 

Geraniol is the primordial monoterpene, and its simple derivatives and cyclization products occur in the oils of many plants, used as flavoring and aromatic agents (e.g., caraway, coriander, dill, eucalyptus, lavender, orange, peppermint, rose, and sandalwood), as well... 

The coupling reaction of epoxide 69 with the anion derived from the allyl sulfide 57, a system derived from geraniol, was accomplished in a 99% yield by employing 4 equivalents of the anion (Scheme 13). Protection of the alcohol as the MOM ether (95%), followed by debenzylation and removal of the phenylthio group under Birch conditions (76%) gave diene 70. Vanadium catalyzed oxidation and spontaneous cyclization of 70 imder acidic conditions afforded a diastereomeric mixture of tetraols 71 and 72 in 61% and 14% yields, respectively. 

Starting with geraniol (126), Sharpless asymmetric epoxidation [68] proceeded in 95% yield and with 93% ee (see Scheme 29) [69]. Hydrobromination of the alkene was achieved under standard conditions using NBS in THF and water (5 1) to afford a 64% yield of diastereomeric bromo alcohols (127). Cyclization to the tetrahydropyran diol was accomplished by treating 127 with camphorsulphonic acid in diethyl ether, followed by diol cleavage with sodium periodate to give a mixture of aldehydes 128 and 129. Upon... 

---