Prolines rearrangement

The thiol 48 (with R R, R = H) undergoes intramolecular cychsation in THF, in the presence of AIBN under UV irradiation, to give a 2-phosphonothiolane [36], a phosphorus and sulfur analogue of proline. More recently, an asymmetric version of the sequence, [2,3]-sigmatropic rearrangement and radical cychsation, has been carried out (see Sect. 5.1.1.) [41]. 

The key step in the total synthesis of (—)-epilupinine 253 involved the ring expansion of a proline-derived spirocyclic ammonium ylide to give 252 through a [1,2] Stevens rearrangement, as shown in Scheme 51 <1997T16565>. 

The bisamide odorine-roxburghilin and its dihydro derivative have been synthesized twice via Curtius rearrangement of a proline azide (127). The two syntheses followed similar lines and differed only in the preparation of the azide and in the solvent for rearrangement (104, 105). The stereospecificity of the Curtius rearrangement was an important argument for the determination of the absolute configuration of odorine (Scheme 23). 

A method for highly efficient asymmetric cyclopropanation with control of both relative and absolute stereochemistry uses vinyldiazomethanes and inexpensive a-hydroxy esters as chiral auxiliaries263. This method was also applied for stereoselective preparation of dihydroazulenes. A further improvement of this approach involves an enantioselective construction of seven-membered carbocycles (540) by incorporating an initial asymmetric cyclopropanation step into the tandem cyclopropanation-Cope rearrangement process using rhodium(II)-(5 )-N-[p-(tert-butyl)phenylsulfonyl]prolinate [RhjtS — TBSP)4] 539 as a chiral catalyst (equation 212)264. 

Rhodium(II) (iV-dodecylbenzenesulfonyl)prolinate has been found to act as an effective catalyst for the enantioselective decomposition of vinyldiazoacetates to c -divinylcyclopropanes. Combination of this process with a subsequent Cope rearrangement has resulted in a highly enantioselective synthesis of a variety of cycloheptadienes containing multiple stereogenic centres (see Scheme 40). The tandem... 

X.J. Wang, S.A. Hart, B. Xu, M.D. Mason, J.R. Goodell, F.A. Etzkorn, Serine-c/s-proline and Serine-frans-proline isosteres Stereoselective synthesis of (Z)- and ( )-alkene mimics by still-wittig and ireland-claisen rearrangements, J. Org. Chem. 

Further elaboration of the sulfur cycloadducts could be achieved by the use of a Pummerer rearrangement in the syntheses of 5-(hydroxymethyl)prolines. Oxidation of adduct 298 to sulfoxide 299, followed by treatment with TEA in DCM and quenching with either methanol or benzyl alcohol, delivered the Pummerer products 300 in 57% yield for R = Me and 38% for R = Bn as single diastereoisomers. Raney Ni desulfurization and Pearlman s catalyst mediated hydrogenolysis, for R = Bn furnished the final enantiopure proline derivative (Scheme 3.99). 

Treatment of proline derivative 154 with rhodium acetate (57) originally led to ylide 155. However, this ylide (155) quickly rearranges to the more stable azomethine ylide 156, which undergoes cycloaddition with DMAD to give the unusual adduct 157. Intramolecular trapping experiments (58,59) have also been conducted (Scheme 4.35). 

The [2,3]-sigmatropic rearrangement of (E)-(218a), a derivative of the chiral cyclic a-amino acid (S)-proline, produced the aminonitrile (219) in a stereoselective manner. Saponification of (219) yielded (+)-2-methyl-2-phenyl-3-butenal (220) with an enantiomeric excess of 90%219>. In replacing the benzyloxymethyl moiety in (218a) by a methyl group, the optical purity of the chiral aldehyde (220) obtained in the corresponding reaction sequence decreases considerably 219). 

Raggatt ME, Simpson TJ, Wrigley SK (1999) Biosynthesis of XR587 (Streptopyrrole) in Streptomyces rimosus Involves a Novel Carbon-to-Nitrogen Rearrangement of a Proline-Derived Unit. Chem Commun 1039... 

Another proline-derived chiral base, namely 17, has been reported by Davidsson and coworkers21. It rearranges cyclohexene oxide 1 into (S)-2 in 78% ee (Scheme 12). 

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