Via intramolecular Diels-Alder

Carbene complexes which have an all-carbon tether between the diene and the dienophile react via intramolecular Diels-Alder reaction to give the corresponding bicyclic compound. The stereoselectivities of these reactions are comparable to those observed for the Lewis acid-catalysed reactions of the corresponding methyl esters and much higher than those of the thermal reactions of the methyl esters which are completely unselective. Moreover, the ris-sub-stituted complexes undergo endo-selective reactions where the corresponding reaction of the ester fails [109] (Scheme 61). 

Keywords chiral lanthanoid complexes, rare earth complexes, carbocyclic ring construction via intramolecular Diels-Alder reaction... 

Thomas E. J. Cytochalasan Synthesis Macrocycle Formation Via Intramolecular Diels-Alder Reactions Acc. Chem. Res. 1991 24 229 235... 

The synthesis of (5 5 5) fused heterocycle 62 has been achieved via intramolecular Diels-Alder reaction involving a series of A-alkenyl-substituted furanyl amides 494 (Scheme 112) <2006JOC5432>. 

The amino carbinol pyridines 264, prepared from 263 by DoM reaction and aliphatic or aromatic carboxaldehyde quench, are useful intermediates for condensed naphthyridines 266-268. The thermolytic reaction proceeds via intramolecular Diels-Alder reactions of the aza-orf/io-quinodimethane species 265 (Scheme 79) (84CC1304). 

The 1,2,4-substituted triazine core is a versatile scaffold to access a wide range of condensed heterocyclic ring systems via intramolecular Diels-Alder reactions with a vast array of dienophiles. The triazine ring system is also a key component of commercial dyes, herbicides, insecticides and also recently appeared in medicinal chemistry. One way to synthesise triazines is to use a three-component reaction that has been described in the literature several times, both under traditional thermal heating and under solvent-free microwave-assisted conditions42. However, the previously described methods focussed only on simple aliphatic phenyl and ester substituents. 

The 1,2,4-triazine core is a synthetically important scaffold because it could be readily transformed into a range of different heterocyclic systems such as pyridines (Sect. 3.1) via intramolecular Diels-Alder reactions with acetylenes. 1,2,4-Triazines have been synthesized by the condensation of 1,2-diketones with acid hydrazides in the presence of NH4OH in acetic acid for up to 24 h at reflux temperature. Microwave dielectric heating in closed vessels allowed the reaction to be performed at 180 °C (60 °C above the boiling point of acetic acid). As a result, the reaction time was reduced to merely 5 minutes. Subsequently, a 48-membered library of 1,2,4-triazines was generated from diverse acyl hydrazides and a-diketones [139]. Two thirds of the desired heterocycles precipitated from the reaction mixture upon cooling with > 75% purity, while the remaining part of the library was purified by preparative LCMS (Scheme 56). 

Products from such reactions have been employed in the synthesis of terpenes via intramolecular Diels-Alder cyclizations (94-96). If the bulk of the carbonyl reagent is increased by inclusion of a large adjacent protecting group, much improved regioselectivity toward terminal attack is acheived (97,98). A complementary approach has been adopted by Oppolzer who... 

TOTAL SYNTHESIS OF BIOLOGICALLY INTRIGUING DRIMANE-TYPE SESQUITERPENOIDS VIA INTRAMOLECULAR DIELS-ALDER APPROACHES... 

Martin SF, Li W (1989) General Methods for Alkaloid Synthesis via Intramolecular Diels-Alder Reactions. A Concise Formal Total Synthesis of ( )-Dendrobine. J Org Chem 54 268... 

An example of an oxaallylic carbanion has been described by Oppolzer et alP In the alkylation of 3-triethylsilyloxypentadienyllithium (5 X = H) the y-alkylated product predominated. With the anion (5 X = SMe) alkylation occurs exclusively at the y-carbon atom. The synthetic potential of the anions (5) was demonstrated by the synthesis of bicyclic compounds via intramolecular Diels-Alder reactions of the alkylation products (Scheme 5). 

Total Synthesis of Biologically Intriguing Drimane-Type Sesquiterpenoids Via Intramolecular Diels-Alder Approaches... 

Vinylcyclopentanone Ig and the corresponding enol silyl ether 12 have been used recently in some elegant, creative, and efficient constructions of estrones via intramolecular Diels-Alder cycloaddition reactions of intermediate -quinodimethanes (Scheme II) (16-18). Only one report, however, has appeared... 

Markgraf JH, Finkelstein M et al (1996) Canthme analogs via intramolecular Diels-Alder reactions. Tetrahedron 52 461-470... 

Scheme 6.24 Bicyclizations via intramolecular Diels-Alder cycloaddition...

Scheme 10 Synthesis of PAH 34 via intramolecular Diels-Alder reaction [31, 32]...

The forth synthetic strategy is to utilize pyrone structure to trigger intramolecular Diels-Alder reaction [30]. In this strategy, we can construct the core structure of Maoecrystal V via compound 1.7.26. As shown in Fig. 1.18, natural product Maoecrystal V can be transformed to compound 1.7.24 via simple functional group transformations, and compound 1.7.24 can be prepared by compound 1.7.25 via ring-opening lactonization. Compound 1.7.25 can be constructed by compound 1.7.26 via intramolecular Diels-Alder reaction of pyrone. The precursor (1.7.26)... 

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