Hetero Diels-Alder, approach

A. Dondoni, L. Kniezo, and M. Martinkova, A stereoselective hetero-Diels-Alder approach to carbon-carbon linked disaccharides, J. Chem. Soc. Chem. Commun. p. 1963 (1994). 

Tetrahydroalstonine 7-7, a heteroyohimboid alkaloid, has been synthesised in enantiopure form by Martin et al. by means of an oxa Diels-Alder reaction as key step. The trienic precursor 7-5 underwent a thermal intramolecular cycloaddition to form a 5 1 mixture of 7-6 and its 15/J-epimer. The main cycloadduct was then subjected to a straightforward sequence to yield the natural product 7-7 (Fig. 7-2) [483-485]. In earlier work, Ogasawara et al. have employed a con-ceptionally different domino Knoevenagel-hetero Diels-Alder approach to this alkaloid and other natural products [486-488]. 

Stereoselective Diels-Alder reactions have been reported in several cases. Enantioselective Diels-Alder reactions of l-phenoxycarbonyl-l,2-dihydropyridine with 1-alkylated acryloyl-pyrazolidin-3-ones using a chiral cationic palla-dium-phophinooxazolidine catalyst afforded chiral isoquinuclidines with excellent enantioselectivity <2005TL5677>. Bismuth(lll) chloride-mediated diasteroselective intramolecular [4-f2] cycloaddition reactions of A-allyl derivatives of pyrazole aldehydes led to fused sulfur-containing pyrazole heterocycles <2003SC3063>. A highly diastereoselective intramolecular hetero-Diels-Alder approach toward tetracyclic pyrazoles from 5-(3-methyl-2-butenylthio)-3-methyl-l-phenyl-4-pyrazolecarboxaldehyde has been reported <1997SL1155>. 

Another interesting example of the preparation of unsaturated O- and C-glycosides 36 was presented recently [44]. This was based on the Sn2 attack of the nucleophile on unsaturated epoxide 35. Substitution at C-1 proceeded with simultaneous migration of the double bond and opening of the three-member ring providing 2,3-unsaturated derivative 36 (O Scheme 20). Conversion of 4-deoxy-2,3-unsaturated sugars into 3,4-unsaturated ones reported by Banaszek and Zamojski already in 1972 [3] was one of the key-steps in total s)mthesis of monosaccharides by a hetero Diels-Alder approach. 

Prabhakaran, J. Lhermitte, H. Das, J. Sasi-Kumar, T.K. Grierson, D.S. The synthesis of a sulfone containing analogue of the esperamicin-A(l) aglycone A hetero Diels-Alder approach. Synlett 2000, 658-662. 

Work in this area has also been carried out with noncyclic carbohydrates. Reaction of the nitro sugar 289 with cyclopentanedienone gave a mixture of 291 and 290 in a 1.9 1 ratio (Scheme 52) [78]. Dondoni used his masked formyl derivative in a hetero-Diels-Alder approach to carbon-linked disaccharides. Cycloaddition of 293 with ethyl vinyl ether gave a mixture of isomers the major compound, 294, is shown in Scheme 52. The cycloadduct was then converted to 295 via standard methods [79]. 

Stanforth and co-workers made additional improvements on the hetero-Diels-Alder approach. They accomplished a one-pot synthesis of pyridines from a p-diketoesters and amidrazones <04T8893>. Deniaud et al. has investigated diazadienium iodide 15 as an aza-diene moiety in the synthesis of pyridines <04TL9557>. They have demonstrated that diazadienium iodide 15 reacts with ketenes, acetylenes and acrylic dienophiles to yield a variety of substituted pyridines as shown in Scheme 5. 

The hetero Diels-Alder approach to carbohydrate-containing molecular scaffolding 07COS47. 

The presented review describes total syntheses directed towards 6-amino-6,8-dideoxy-D-eryt/iro-D-galacto-octose, commonly named lincosamine - the sugar component of the antibiotic lincomycin. In the first part we present total syntheses of lincosamine that start from carbohydrate precursors. The D-galactose-derived aldehyde is the most frequently used synthon. In the second part, total syntheses of lincosamine from non-carbohydrate precursors are presented. This part of the review is divided into two subsections. The first one groups syntheses based on the application of furan compounds. In the second one we present a hetero-Diels-Alder approach to the synthesis of lincosamine. 

Hetero-Diels-Alder Approach to the Synthesis of Lincosamine. 377... 

Lucas BS, Luther LM, Burke SD. A catalytic enantiose-lective hetero Diels-Alder approach to the C20-C32 segment of the phorboxazoles. J. Org. Chem. 2005 70 3757-3760. 

Crimmins, M.T. and Smith, A.C. (2006) A hetero-Diels-Alder approach to complex pyrones an improved synthesis of the spongistatin AB spiroketal. Org. Lett., 8,1003-1006. 

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