Mannich diene syntheses with

Mannich ketobases may also be useful materials for the synthesis of O-hetcrocycles (Fig. 134), as they behave as masked heterodienes in the reaction with dicnophiles. The a,P-unsaturatcd ketone formed by elimination of the amino group can in fact undergo cycloaddition by the dienophile to give the dihydro-4H-pyrans 346. In the event that an HX molecule is eliminated, the pyran ring is formed. - - - The above reaction is also quite favored by dienes, and it can be competitive with the diene synthesis shown in Fig. 129. 

A similar approach to the synthesis of tetracyclic indole alkaloid derivatives has been described [182], and the use of reactive chiral iminium ions allows the realisation of stereoselective aza Diels-Alder reactions even in aqueous solution [183,184]. Nevertheless it should be noted that reactions of electron-rich dienes with imines e. g. derived from amino acids do not necessarily proceed via a Diels-Alder mechanism. They may as well undergo a domino-Mannich-Michael sequence which also efficiently leads to useful nitrogen heterocycles [185-188]. 

A variety of carbonyl reactions can be performed with aldehyde groups adjacent to (diene)iron complexes. This includes cyanohydrin formation as part of a synthesis employing sequential stereoselective introduction of nucleophiles to (diene)iron complexes with concomitant elongation of the carbon chain (see also Scheme 4-155). 370,371,374,375] reactions of aldehydes adjacent to (diene)iron complexes with methyl ketones have been reported. Diastereoselective intramolecular Mannich reaction of a... 

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