Hetero Diels-Alder-type cycloadducts

It has been reported that Cplex-isoelectronic theory predicts endo selectivity in Diels-Alder reactions. Phosphorylated nitroso alkenes react with cycUc dienes to give hetero Diels-Alder-type cycloadducts and subsequent aza-Cope rearrangement gives 5,6-dihydro-4H-l,2-oxazines (Scheme 20). ... 

Keck also investigated asymmetric catalysis with a BINOL-derived titanium complex [102,103] for the Mukaiyama aldol reaction. The reaction of a-benzyloxyalde-hyde with Danishefsky s dienes as functionalized silyl enol ethers gave aldol products instead of hetero Diels-Alder cycloadducts (Sch. 40) [103], The aldol product can be transformed into hetero Diels-Alder type adducts by acid-catalyzed cyclization. The catalyst was prepared from BINOL and Ti(OPr )4, in 1 1 or 2 1 stoichiometry, and oven-dried MS 4A, in ether under reflux. They reported the catalyst to be of BINOL-Ti(OPr% structure. 

Phosphonyl and phosphinyl nitroso alkenes (607) reacted with cychc conjugated nucleophilic dienes such as cyclopentadiene or cyclohexadiene (608) and silyl-substituted cyclopentadiene (609) to afford [4+2]-hetero Diels-Alder-type 1,2-oxazines cycloadducts (610) and (612) The nitroso alkenes participated as dienophile components and the cyclic olefins acted as the 471-electron (diene) systems. Cycloadducts (611), derived from cyclopentadiene, underwent a new aza-Cope type [3,3]-sigmatropic rearrangement to give 1,2-oxazines (612) (Scheme 150). ... 

The diversity of the products obtained by the three-component domino-Knoevenagel-hetero-Diels-Alder reaction can be further increased by a different work-up of the formed cycloadduct 141. Thus, hydrogenolytic removal of the Cbz-group in 141 led to 151 with a lactam and an aldehyde moiety by reaction of the formed secondary amine with the lactone moiety followed by elimination of benzyl alcohol. Reduction of 151 with lithium aluminum hydride gave benzoquinolizidine 152 (Scheme 5.30). Alkaloids of this type have so far not been found in nature, but it can be assumed that they might exist, since they could easily be formed from deacetylisopecoside 153, which is an intermediate in the biosynthesis of emetine 111. 

Numerous studies have dealt with different types of sulfur-containing hetero-dienophiles. Thus, hetero Diels-Alder reactions of N -sulfinyl dienophiles have been thoroughly studied by Weinreb et al. [454] the resulting cycloadducts represent useful and versatile intermediates in the synthesis of homoallylic amines [455] or pyrroles [456]. Further work using this type of S = N dienophiles... 

A novel sterically congested cliv-like receptor for dicationic quests, incorporating a hindered cyclophane possessing chiral spirobiindanol phosphonate and phosphate units (32) has been synthesised P-Nitrosophosphates (33) have been obtained and used as new N-O hetero-dienophiles in the Diels-Alder type reaction with a few selected 1,3-dienes to give highly functionalized cycloadducts (34). The latter can be directly transformed by... 

The enantioselective total synthesis of the Securienega alkaloid (-)-phyllanthine by S.M. Weinreb et al. involved a stereoselective Yb(OTf)3-promoted hetero Diels-Alder reaction between a cyclic imine dienophile and Danishefsky s diene This was the first example of using an unactivated cyclic imine in this type of cycloaddition. Commonly used Lewis acid catalysts (e.g., SnCb, TiCU, etc.) produced only low yields of the desired cycloadduct. However, it was discovered that ytterbium triflate catalyzed the cycloaddition and afforded the product in 84% yield. Later they also found that the cyclization could occur at high pressure and in the absence of the catalyst, although a slightly lower yield (71%) of the product was obtained. 

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