Ethylidene triphenylphosphorane

EthytidenationThe ketone (2) undergoes Wittig reaction with ethylidene-triphenylphosphorane in yields of only about 40%, with recovery of 2. The difficulty may be enolization promoted by the basic ylide. Use of the Grignard reagent from a-chloroethyltrimethylsilane results only in reduction of the carbonyl group. The problem is solved by use of 1, which reacts with 2 to give the internal ketal 3. Hydro-... 

The same synthetic route was applied to the preparation of the tricyclic sesquiterpene santalol 235). The reaction of the tricyclic aldehyde 435 with ethylidene-triphenylphosphorane and paraformaldehyde 205 gives the a,P-unsaturated alcohol 436, elimination of phosphorus being probably directed by the thermodynamical stability of 436 235) (Scheme 76). 

Trans P-sinensal 441 is obtained from the aldehyde 438 prepared by selective ozonolysis of trans-famesene 437. Wittig olefmation of 438 with 1-formylethylidene-phosphorane 440 gives trans P-sinensal 441. Reaction of 438 with 1-ethoxycarbonyl-ethylidene-triphenylphosphorane 439 yields the ethyl ester 442, from which 441 can also be obtained 237 (Scheme 77). Furthermore, a working group of the BASF synthesized a-sinensal 443 and p-sinensal 441 using Wittig reactions 238). 

To a one-necked round-bottomed flask (10 mL) equipped with a magnetic stirrer, water condenser and under a nitrogen atmosphere, add (2S,3/7)-3-[(triethylsilyl)oxy]-2-methylhexanal (168 mg, 0.678 mmol), I1-(ethoxycarbonyl)ethylidene]-triphenylphosphorane (299 mg, 0.743 mmol) and dry dichloromethane (5 mL). 

Similar effects are observed for phosphonium ylides18 (see also Vol. El, p 704) which are able to transfer their alkylidenc unit to a,/ -unsaturated esters. The ring closure of the intermediate betaine proceeds under steric control and preferentially generates the less hindered cyclopropane derivatives, for example, ethylidene triphenylphosphorane and methyl (E)-2-butenoate furnish a 93 7 mixture of methyl 2,3-dimethylcyclopropanecarboxylate 519. 

The first one, reported by Szarek and co-workers [16], starts with the Wittig reaction between aldehyde 10 and ethylidene triphenylphosphorane [17] (Scheme 2), affording the cis-isomer 11 as a major product. 

Readily available aldehyde (98) could be converted to a,p-unsaturated aldehyde (99) in three simple steps. This compound on treatment with ethylidene triphenylphosphorane at low temperature, followed by n-butyl... 

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