A general method for the palladium-catalyzed formation of diaryl ethers is described. Electron-rich, bulky aryldialkylphosphine ligands, in which the two alkyl groups are either tert-butyl or 1-adamantyl,
are the key to the success of the transformation. A wide range of electron-deficient, electronically neutral and
electron-rich aryl bromides, chlorides, and triflates can be combined with a variety of phenols with the use of
sodium hydride or potassium phosphate as base in toluene at 100 °C. The bulky yet basic nature of the phosphine
ligand is thought to be responsible for increasing the rate of reductive elimination of the diaryl ether from
palladium.
[equation--see text] The N-arylation of indoles, including a variety of substituted ones, has been carried out using bulky, electron-rich phosphines as the supporting ligand in combination with Pd(2)(dba)(3). Using this catalyst system, the efficient coupling of indole and a variety of substituted indoles with aryl iodides, bromides, chlorides, and triflates can be achieved.
Intramolecular Heck reactions of alpha,beta-unsaturated 2-haloanilides derived from azatricyclo[4.4.0.0(2,8)]decanone 5 efficiently install the congested spirooxindole functionality of gelsemine. Depending upon the Heck reaction conditions and the nature of the beta-substituent, either products having the natural or unnatural configuration of the spirooxindole group are formed predominantly. Efforts to elaborate the hydropyran ring of gelsemine from the endo-oriented nitrile substituent of pentacyclic Heck product 18 were unsuccessful. Important steps in the ultimately successful route to (+/-)-gelsemine (1) are as follows: (a) intramolecular Heck reaction of tricyclic beta-methoxy alpha,beta-unsaturated 2-iodoanilide 68 in the presence of silver phosphate to form pentacyclic product 69 having the unnatural configuration of the spirooxindole fragment, (b) formation of hexacyclic aziridine 80 from the reaction of cyanide with intermediate 79 containing an N-methoxycarbonyl-beta-bromoethylamine fragment, (c) introduction of C17 by ring-opening of the aziridinium ion derived from aziridine 80, and (d) base-promoted skeletal rearrangement of pentacyclic equatorial alcohol 82 to form the oxacyclic ring and invert the spirooxindole functional group to provide hexacyclic gelsemine precursor 83.
A complex molecular reorganization (1-->2), a sequential anionic aza-Cope rearrangement and Mannich cyclization, and an unprecedented intramolecular Heck reaction of the tetrasubstituted double bond of a vinylogous carbamate are key steps in a new total synthesis of (+/-)-gelsemine (3). MOM=methoxymethyl, DBU=1,8-diazabicyclo[5.4.0]undec-7-ene.
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