Studies on the Highly Selective Reactions of Functional Methylenecyclopropanes
|Keywords||Alkylene Reaction selectivity Cyclopropane derivatives Functionalized Cyclopropanation Three-membered ring Lewis acid catalyzed Furanone Organic Synthesis equiv Compounds phenyl Double bond Stereoselectivity Cycloaddition Ring expansion reaction Pinacol rearrangement Transition metal-catalyzed Ring contraction reaction Ring expansion rearrangement|
The development of novel reactions with high regio- and stereoselectivity for construction of complex molecules is a hot topic in organic synthesis. Recently, we have witnessed tremendous growth in the strategies converting easily accessible starting materials to complex and interesting organic molecules.Methylenecyclopropanes (MCPs) are highly strained but readily accessible carbocyclic molecules that have served as useful building blocks in organic synthesis. In the past decades, mounting attention has been paid to the transition metal- or Lewis acid-catalyzed reactions of MCPs, which have been employed for the construction of complex and interesting organic molecules. However, an attractive but often troublesome feature of MCPs is their multi-forms of reactivities leading to the formation of a variety of different products. Thus, controlling the regio- and stereoselectivity in reactions of MCPs is a formidable challenge in organic synthesis.Recently, MCPs with functional groups attached to a cyclopropyl ring or C=C bond have received considerable attention. In principle, the presence of functional groups may facilitate the selective cleavage of C-C bonds of MCPs, thus delicately tuning the regio- and stereoselectivity of the reactions.We have developed the following reactions:Firstly, we disclosed the ZnCl2-mediated reactions of (E)-2-(arylmethylene)-cyclopropylaldehyde with various acyl chlorides providing a novel method for stereoselective synthesis of bifunctional methlylenecyclobutanes via a proximal-bond cleavage process. Nevertheless, when (Z)-2-(arylmethylene)cyclopropylaldehyde was employed, the reactions gave 1,3-conjugated dienes through distal-bond cleavage. On the other hand, we realized an interesting route to cyclopentenones from bifunctional methlylenecyclobutanes via a process of epoxidation, pinacol-type ring enlargement and subsequent elimination.Secondly, we have observed a ring-opening intramolecular radical cyclization and an electrophilic cyclization reaction of (E)-2-(arylmethylene)cyclopropylaldehyde, affording a controlled synthesis of 1-naphthaldehydes and 3-oxabicyclo[3.1.0] hexan-2-ols. The properties of organo-selenium reagents such as PhSeSePh, N-PSP, PhSeBr and PhSeCl may play an important role in the reactions.Thirdly, we have developed a cyclization of 3-cyclopropylideneprop-2-en-1-ones in the presence ofπ-philic metals leading to furan-fused cyclobutenes, which is a versatile intermediate for further elaboration. Application of PdCl2 with additional oxidant DMP or CuCl at 50℃led to a highly selective formation of the functionalized 2-alkylidenecyclobutanones and 6,7a-dihydrobenzofuran-7(3aH)-ones with one spiro-three-membered ring and one four-membered ring from cyclopropylideneallenyl ketones directly. Furthermore, we provided a clean route to 2(3H)-furanones with a spiro-three-membered ring at the 3-position from 2-alkylidenecyclobutanones. Possible mechanism is proposed on the basis of the series of NMR experiments which lead to the observation of key intermediates.We have also found the phosphor-hydroxylation reaction of 3-cyclopropylidene-prop-2-en-1-ones. On the basis of O18 labeling experiment, a plausible reaction mechanism was proposed.