Spiroannulated cyclopropanes

Reductive dehalogenation is an efficient method of synthesis of cyclopropanes spiroannulated to five- and higher-membered carbocycles (i.e. compounds in which spiroannulation does not result in accumulation of extra strain) . The required gem-(dihalomethyl)cycloalkanes are usually prepared by halogenation of the precursor diols (equation 1). The cyclization is most efficiently accomplished in the Zn-alcohol-water system . For example, spiro[2.5]octane 7 was prepared in 91% yield using this procedure. This method is useful even for a one-step preparation of bis-spirocyclopropyl compounds as exemplified in equation 2. However, the application of the reductive dehalogenation method to the synthesis of more strained SPC (i.e. spirohexane or spiropentane) often leads to rearranged products. For example, methylenecyclopentane was the only product obtained from bis(bromomethyl)cyclobutane (equation 3) ... 

The cyclopropanated furanone 32 was found to behave as a Diels-Alder diene. Thus, 32 reacts with maleic anhydride (33) to give a tetracyclic adduct 34 which is formed endoselectively (Scheme 9) (93SL415). The latter adduct is a spiroannulated butenolide related to naturally occurring sesquiterpenes [85AG(E)94 85TL5453],... 

VI. CONJUGATION BETWEEN CYCLOPROPYL FRAGMENTS AND ITS INFLUENCE ON CHEMICAL PROPERTIES OF SPIROANNULATED CYCLOPROPANES. 870... 

TABLE 1. Calculated (MM2) and experimental heats of formation and strain energy in some spiroannulated cyclopropanes... 

Diels-Alder cycloadditions of the readily available spiro[2.4]heptadiene (15) are a useful entry to spiro(cyclopropane-l,7 -norbornene) and related polycyclic compounds (equation 17)38. 1,1 -Diacetylcyclopropane was found to undergo unusual base catalyzed cyclizations resulting in spiroannulated cyclopropanes (equation 18)3,-4°. 

V. NON ACTIVATE D SPIROANNULATED CYCLOPROPANES AND SOME OF THEIR USEFUL APPLICATIONS... 

It was demonstrated in Section 3 that spiroannulation of cyclopropane to four- and higher-membered rings does not cause an increase in the strain energy of the molecule. It is not surprising, then, that the chemistry of these compounds does not differ from that of simple alkyl substituted cyclopropanes. Since exciting reviews on the chemistry of cyclopropanes are available42, the authors of this chapter will limit the description of properties of SPC of these types to some of their interesting applications. 

As demonstrated in Section IV, compounds possessing two spiroannulated cyclopropyl fragments could be successfully prepared by using double cyclopropanation or double cyclization of the corresponding bismethylene or bis(gem-dihalomethyl) precursors. 

Triangulanes are a unique class of polycyclic hydrocarbons constructed from spiroannulated cyclopropanes. Because of a stereochemical diversity, the number of isomeric triangulanes sharply increases with increasing the number of cyclopropyl fragments. Consequently, one may speak about the land of triangulanes . 

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