Organocatalytic Asymmetric Synthesis of Spiroacetals and Bridged Acetals

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Chiral spiro compounds have been found to have great importance in organic synthesis because of their presence in a variety of natural alkaloids and pharmaceuticals. Lately, spiro compounds have gained interest because of their engrossing conformational characteristics and their structural connection with biological systems. Enantioselective synthesis of conformationally constrained spiro and bridged acetals can also be performed by organometallic catalysts but we will focus on organocatalytic routes in this chapter. Organocatalytic methodologies are found to be powerful approaches for the synthesis of conformationally rigid spiro and bridged acetal compounds because of their stability, functional group tolerance and easy stereoprediction. In this chapter, the organocatalytic asymmetric approaches for the synthesis of bridged [2.2.1], [2.2.2], [3.1.1], [3.2.1], [3.3.1] bicyclic acetals as well as spiroacetals are discussed in details with examples. The synthesis contains different reactions such as Michael addition reaction, Mannich reaction, cycloaddition reaction, aldol reaction, tandem Friedel Craft/hemiketalization reaction, Knoevenagel, Diels Alder reaction, cyclisation reaction and various other reactions.