PAQUETTE Olefination

PAQUETTE Olefin Synthesis OesuHonatlon of suHones to alkenes (altemative to Ramberg-Backlund). 

PAQUETTE Olefin Synthesis Desulfonation o( sutfones to alkenes (alternative to Ramberg-Backlund). 

Treatment of a-halosulfones 357 bearing a -hydrogens with base gives olefins by extruding SO2 from the episulfone intermediates 359. The reaction was found by Ramberg and Backlund in 1940 and named after them, and it has been very useful for the preparation of tailor-made olefins. The reaction has been investigated in detail " and utilized widely for olefin syntheses. An excellent review on this reaction by Paquett covers the literature up to 1975, so only recent studies are dealt with in this section. The generally accepted mechanistic path is shown below. 

CONTENTS List of Contributors. Introduction to the Series An Editor s Forward, Albert Padwa. Preface, Randolph P. Thummel. Cyclooctatetraenes Conformational and ii-Elec-tronic Dynamics Within Polyolefinic [8] Annulene Frameworks, Leo A. Paquette. A Compilation and Analysis of Structural Data of Distorted Bridgehead Olefins and Amides, Timothy G. Lease and Kenneth J. Shea. Nonplanarity and Aromaticity in Polycyclic Benzenoid Hydrocarbons, William C. Herndon and Paul C. Nowak. The Dewar Furan Story, Ronald N. Warrener. Author Index. Subject Index. 

Capnellane is the generic name applied to a group of sesquiterpene alcohols and the hydrocarbon isolated from the soft coral Capnella imbricata A < >-Capnellene (667), the presumed biosynthetic precursor of the capnellenols, was first synthesized in 1981 by Stevens and Paquette Their synthetic plan called for the construction of bicyclic ketone 668 and its appropriate annulation. The latter event was achieved by application of the Rupe rearrangement to 668, conjugate addition of a vinyl group to 669, ozonolysis, and cyclization (Scheme LXVIII). Hydrogenation and olefination completed the sequence. 

Selective chemical transformations can also be performed on the natural product sanglifehrin A (1) itself [19]. It is surprising that C26-C27 of 1 can be chemoselectively cis-dihydroxylated (Sharpless conditions) in a yield of 70%. The natural product can afterwards be reassembled by Julia-Kocienski olefination. The success of this operation indicates that total syntheses of sanglifehrin A (1) alternative to those by Nicolaou et al. and Paquette should be worth pursuing. 

A particularly demanding constitutional problem in the context of a bridgehead olefinic system was solved by Paquette using the ME rearrange-... 

Paquette and coworkers [127], in a synthesis of the complex polyol antibiotic amfidinolu-3, made masterly use of the Julia-Kodenski olefination reaction. Two examples taken from that work are presented below. Reaction of sulfone 285 with aldehyde 284 (Scheme 92) carried out with KHMDS in THF (at - 78 °C to rt) gave the building block 286 with 90% yield but with relatively poor selectivity, EjZ 1>I. Free-radical isomerization of the mixture (benzene, AIBN, reflux) increased the isomer ratio, /2 6/1. 

Lobben, P. C., Paquette, L. A. Sequenced reactions with samarium(ll) iodide. Tandem nucleophilic acyl/ketyl-olefin coupling reactions. Chemtracts 1997, 10, 284-288. 

Hong, F.-T., Paquette, L. A. Olefin metathesis in cyclic ether formation. Direct conversion of olefinic esters to cyclic enol ethers with Tebbe-type reagents. Copper(l)-promoted Stille cross-coupling of stannyl enol ethers with enol triflates construction of complex polyether frameworks. Chemtracts t997, 10,14-19. 

Support for this mechanism came from an unexpected quarter. In a synthesis of sterpuric acid 215, Paquette planned to put in the alkene at the end by a Julia olefination.35 Addition of MeLi to the ketone 216 and non-stereospecific elimination (chapter 15) looked a good strategy. The 1,2-relationship between OH and ketone in 216 is awkward and the decision was made to try a hydroxylation on the silyl enol ether of the parent ketone 217. 

This sequence has been shown because there are examples known of substituted olefins being cyclized to cyclobutane derivatives by certain transition metal complexes (Schrauzer Heimbach, Jolly and Wilke °W). In addition, metal complexes will catalyze the opening of cyclobutane rings to olefins and other rearranged products, as shown particularly by Cassar, Eaton and Halpern f). It is likely that the sequence shown in Fig. 15, both forward and reverse, is the mechanism for many of these reactions. Metal ions such as silver(I) which do not undergo oxidation readily probably involve a different mechanism (Paquette 2a) Gassmann 2b)). 

Paquette and Leichter have reported the first case in which a highly stereoselective cycloreversion of a cyclobutane to two olefins becomes energetically accessible, as a result of suprafacial participation of a third proximate o-bond. Thermolysis of the ant/-tricyclo[3,2,0,0 Jheptane (641) gives the cyclopentadiene (642) and olefin (643). The latter had a cis.trans ratio of 90 10, reflecting kinetically controlled product distribution. Biradicals are ruled out by the observed stereoselectivity and the two cyclobutane bonds must be broken more or less simultaneously. Paquette and Leichter favour a + A) process. 

Taylor and Paquette have used the acetone sensitized photochemical rearrangement of bicyclo[3,2,0]hept-6-ene-2-one to tricyclo[4,l,0,0 ]heptan-3-one in an improved synthesis of the derived tricyclic olefin. ... 

In this reaction, an a-halogensulfone 44 is treated with a strong base to give an olefin 46, as illustrated in Scheme 7.16 (Paquette 1968). 

Mercaptans which have at least one a-H-atom can be readily converted in good yields to terminal olefins containing one additional G-atom.— E Cyclohexyl mercaptan treated with paraformaldehydde and anhydrous HCl according to L. A. Walter, L. H. Goodson, and R. J. Fosbinder, Am. Soc. 67, 655 (1945) chloromethyl cyclohexyl sulfide (Y 90%) added dr op wise at —5 to —10 during 15 min. to a stirred soln. of m-chloroperoxybenzoic acid in chloroform, allowed to warm to room temp, and to stand overnight chloromethyl cyclohexyl sulfone (Y 97.0%) stirred and refluxed 24 hrs. with aq. 25%-NaOH methylenecyclo-hexane (Y 80%). Also 1-heptene from 1-hexanethiol s. L. A. Paquette, Am. Soc. 86,4383 (1964). 

In the absence of a directing group, the cyclopropanation of cyclic olefins is generally under steric control. The stereochemical preference can be predicted from the ground state conformation of the molecule and often high levels of stereocontrol are observed. For example, Paquette and coworkers used a Simmons-Smith reaction in their total synthesis of the secondary marine metabolite (+)-acetoxycrenulide (30), whereby high P-face selectivity was observed. ... 

Gentil, S. Dietz, M. Pirio, N. Meunier, P Gallucci, J.C. Gallou, F. Paquette, L. A. A silylene-bridged (isodicyclopentadienyl)(fluorenyl) complex of zirconium for homogeneous olefin polymerization. Organometallics 2002, 21, 5162-5166. 

Allyl vinyl ethers (66) undergo [l,3]rearrangements to give cyclopentanones (67) when heated with Pd° catalysts,in vivid contrast to the uncatalysed thermal rearrangement of compounds such as (66), which results in the formation of cycloheptenones. The recently developed oxyanionic Cope rearrangement of cyclononatrienols has been used by Paquette and Crouse as the key step in a brief synthesis of ( )-multifidene (68)/ Highly substituted alkylcyclopentenes may be prepared by [3 + 2]cycloadditions between allyl cations and olefins/ ... 

Magnesium and methanol, a reducing system described as early as 1929, has now been shown to reduce the carbon-carbon double bond of a,/3-unsaturated amides in the presence of other olefins e.g. Scheme 8). ° The double bond may be mono-, di-, tri-, or tetra-substituted with alkyl or aryl groups, and nitrogen is also optionally substituted. Paquette and his co-workers have extended the... 

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