Buchi s synthesis

Irradiation of 231 gives, after the re ra-Mannich fragmentation of the photoadduct 232, a 91% yield of 233. The single stereogenic center in the photosubstrate leads to complete stereochemical control in the formation of the cyclobutane intermediate 232, which contains two new strereogenic centers. Treatment of 233 with lithium diisopropylamide, followed by an excess of ier butyldimethylsilyl triflate and reaction of the crude product with tetrabutylammonium fluoride, results in the formation of the desired tetracyclic product 234 in 51% yield. This compound is converted to tetracyclic ketone 235, which is an advanced intermediate in Buchi s synthesis of vindorosine. 

Scheme 2. BUchi s radical cyclization strategy for the synthesis of dihydroagarofuran (15).

The third and in many ways most creative phase of George Buchi s career had its roots in his doctoral work at ETH, where he had undertaken the synthesis of degradation products of certain triterpenes. Synthetic studies on natural products has always flourished in George s laboratory at MIT, but with his growing interest in complex alkaloid came the opportunity to ex-... 

Buchi s shorter synthesis of ( )-camptothecin (Scheme 34) consists essentially of a five-stage synthesis of the pyridone (239), which has previously been converted into camptothecin in two stages by other workers. 

The synthetic achievement of the year in this field is certainly the synthesis of loganin penta-acetate (95) by Buchi et al. (Scheme 5). ° Using an extension of de Mayo s method for the synthesis of -diketones by photochemical cycloaddition of enolized /S-diketones to olefins, Buchi s group constructed the... 

The synthesis starts from norbornanone (6.37). This is methylated using iodomethane with sodamide as the base. Alkylation of methyl-norbornanone (6.38), norbornanone (6.37) with Buchi s bromide (6.39) (prepared by addition of hydrogen bromide to acrolein in ethylene... 

The second step of Buchi and Wuest s synthesis of Ambrox involves the cyclisation reaction shown in Figure P26. Suggest a mechanism for this reaction. 

The reaction shown in Figure P27 occurs further along in Buchi and Wuest s synthesis. What is the mechanism of this reaction ... 

BFg-etherate added slowly to an ice-cold soln. of the startg. trans,trans-methyl ketone in methylene chloride, and stirred 15 hrs. at room temp. product. Y 70%. - This ring closure is part of a stereospecific synthesis of alkaloids. Also isomer s. G. Buchi, S. J. Gould, and F. Naf, Am. Soc. 93, 2492 (1971). 

SCHEME 13.7 Buchi s biomimetic synthesis of dibromophakellin and conversion of 44 to dibromoisophakellin by Horne. 

The automation of laboratory operations to enhance the process development chemist s armory has also grown as an activity. Equipment is available (e.g., Buchi Syncore8) to aid in the evaluation and optimization of such as time-temperature cycles in a given reaction, or in combinatorial and parallel synthesis endeavors. 

BUchi has used Mander s approach as a key step in his synthesis of a-sinensal (212 Scheme 50), and Corey has applied the ylidic 3,2-rearrangement to the efficient preparation of 3-cyclopentenones. Thus, base treatment of the sulfonium salt (213) first gave the rearranged product (215). Thermal vinylcyclo-propane ring expansion then produced the spiro compound (216). Deprotection finally gave the noncon-jugated cyclopentenone (217 Scheme 51). 

As was the case with aspidospermine, the first synthesis of vindoline, by BUchi and collaborators 311,312), has been widely studied, and several new syntheses of the critical intermediates 529 and 530 have been reported. Takano s two approaches to these intermediates were discussed in Volume 17 (/). A second preliminary communication on the same theme followed (313), and details were published in 1979 (314). 

Porco, J. A. and Schreiber, S. L., The Paterno-Buchi reaction, in Comprehensive Organic Synthesis, Vol. 5, Trost, B., Ed., Pergamon Press, Oxford, 1991,151. 

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