Cycloadditions rings

Scheme 79 Snapper s total synthesis of asteriscanolide (116) by sequential intramolecular cyclobutadiene cycloaddition, ring-opening CM (ROCM), and Cope rearrangement [167]...

Scheme 2.146. Synthesis of several natural products via Pummerer-type rearrangement/ 1,3-dipolar cycloaddition/ring-opening reactions.

Both reactions have been utilized to prepare heterocyclic compounds such as pyrazoles 681 (X = A -Ph) and isoxazoles 681 (X = O) as shown in Scheme 7.214. " Starting from an unsaturated 5(4//)-oxazolone 677, either a cycloaddition-ring-opening reaction sequence (677 678 680) or a ringopening-cycloaddition reaction sequence (677 679 680) affords the same product. 

Group two, referred to as cycloaddition rings. The ring may be heterocyclic... 

Upon reaction with ynamines 2-methyl-7-tosylfuro[2,3-4]pyridazine 21 undergoes [2-1-2] cycloaddition-ring-expan-sion reactions to afford 22 (Equation 7) <1996H(43)199> as one of three products isolated. 

Kanematsu and co-workers devised a simple pathway to isobenzofurans and dihydroisobenzofurans starting with substituted furans of type 62. Treatment with strong base results in an alkyne-allene isomerization. Subsequent intramolecular cycloaddition, ring opening (probably oxygen lone-pair assisted), and acidic workup give 63. 

Figure 19. Michael addition versus cycloaddition/ring opening reaction manifold for ri2-pyrrole complexes.

Limanto, J., Snapper, M. L. Sequential Intramolecular Cyclobutadiene Cycloaddition, Ring-Opening Metathesis, and Cope Rearrangement Total Syntheses of (+)- and (-)-Asteriscanolide. J. Am. Chem. Soc. 2000, 122, 8071-8072. 

The catalytic system has been efficiently applied to the sequential [4 + 2] cycloaddition/ring opening reaction to afford highly enantioenriched a-hydroxylated carbonyl derivatives in excellent yields (Scheme 10.7). A variety of important classes of compounds, such as a-hydroxy-y-lactone, a-hydroxy-y-lactam, factor Xa inhibitor,... 

Alcaide B, Almendros P, Alonso JM, Redondo MC (2003) Asymmetric synthesis of unusual fused tricyclic P-lactam structures via aza-cycloadditions/ring closing metathesis. J Org Chem 68 1426-1432, and references therein... 

Scheme 6 The redox non-innocent 2,6-diiminepyridine ligand acts as an electron reservoir in Fe-catalyzed [2 + 2] cycloaddition ring-closure reactions...

B. Polar Cycloaddition-Ring-Enlargement Reactions with Acetylenic Esters. . 352... 

VIII. 2-(Triphenylphosphorylidenamino) Esters. The Cycloaddition-Ring Enlargement Sequence... 

Cycloaddition, ring-opening, and other novel reactions of thiophenes , Iddon, B., Heterocycles, 1983, 20, 1127 Cycloaddition reactions wiOi vinyl heterocycles , Sepulveda-Arques, J., Abarca-Gonzalez, B. and Medio-Sim6n, M.,Adv. Heterocycl. Chem., 1995, 65, 339. 

Heterocycles with a similar 1,4-dihydropyridine ring, such as TV-substituted 1,4-dihydroquinolines (39), have also been allowed to react with dimethyl acetylenedicarboxylate. Depending on the substituent at the ring, a (2 + 2)-cycloadduct (40)60 or a linear Michael adduct (41)59 was formed. The (2 + 2)-cycloadducts (43) of l,2-dihydropyridines(42) with dimethyl acetylenedicarboxylate are far less stable. Only NMR spectroscopy at —10° to 0° has provided evidence for the formation of 43. At room temperature the (2 + 2)-cycloaddition was followed by isomerization to the corresponding 1,2-dihydroazocine (44).15>6 The reaction took a different course when other dienophiles were employed for instance, with iV-phenylmaleimide or maleic anhydride, Diels-Alder-type adducts were formed. Reaction of a 1,2-dihydropyrazine (45) with dimethyl acetylenedicarboxylate yielded a bicyclic compound, which was shown to be not the expected (2 + 2)-cycloadduct 46, but the isomeric 2,7-diazabicyclo(4.2.0]octa-2,4-diene (47). This compound was claimed to result from initial (2 + 2)-cycloaddition, ring opening, and subsequent m/ramolecular (2 + 2) cycloaddition [Eq. (9)1.62... 

Limanto. J. Snapper. M.L. Sequential intramolecular cyclobutadiene cycloaddition, ring-opening metathesis, and cope rearrangement Total syntheses of (-i-)- and... 

Cycloaddition, ring-opening, and other novel reactions of thiophenes , Iddon, B.,... 

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