Intermediate compounds polycyclic molecules

The cyclic ylide intermediate 366, as a 1,3-dipole, is generated by intramolecular reaction of Rh-carbene with the ketone in 365, and undergoes cycloaddition with n-bonds to give the adduct 367 [121]. When a-diazocarbonyls have additional unsaturation, domino cyclizations occur to produce polycyclic compounds. The Rh-carbene method offers a powerful tool for the construction of complex polycyclic molecules in short steps, and has been applied to elegant syntheses of a number of complex natural products. 

Gassman reported the photoaddition of nucleophiles to radical cations of highly strained aliphatic polycyclic molecules such as tricyclo[2.2.1.0 ]hexane and related compounds. The radical cation of bicyclo [1.1.0] butane is postulated as a key intermediate (Scheme 12) [45-46]. 

The synthesis of bicyclic molecules containing guanidinium subunits, such as 156 (Scheme 22), are of considerable interest due to the wide range of biological activities presented by this family of natural products (see Section 11.11.9). In one of the first biomimetic studies toward ptylomycalin A, a series of polycyclic compounds have been prepared through an intermediate l-imino-hexahydropyrrolo[l,2-f]pyrimidin-3(4//)-one such as 155. Succinaldehyde... 

The reactions with a.p-unsaturated carbonyl compounds also lead to cyclobutenes (2.102), and there is evidence that, in some cases at least, the mechanism is non-concerted and goes through biradical intermediates that can be trapped by a second molecule of the conjugated alkene (2.103). Intramolecular photocydoadditions offer routes to polycyclic structures, and the cyclobutene unit in the product provides a basis for subsequent chemical transformations such as oxidation 12.104). 

Such a molecule can be stabilized by a system of delocalized Ji-electrons, which is closed into a toroid of 10 aromatic rings. Reactive sites are four CH groups, which are at the ends of this molecular tube. Such substances belong apparently to a new class of organic compounds, which is intermediate between planar polycyclic aromatic hydrocarbons and three-dimensional fullerenes, nanotubes. Quantum-chemical calculations of the electronic and spatial structure of C32H8 and some other molecules indicate that they have an increased reactivity and semiconductor properties. 

The synthesis of enantiomerically pure compounds is the challenging problem for organic chemists. The synthesis becomes obsolete if the intermediates produce racemic mixtures. The problem is particularly acute when the asymmetric centers do not reside in a rigid cyclic or polycyclic framework. To be able to carry out efficient syntheses of complex molecules, chemists have to control the sense of chirality at each chiral center as it is introduced in the course of synthesis. Monoalkyl- or dialkyl-boranes exhibit a remarkable chemo-, stereo-, and regioselectivity for the hydroboration of unsaturated compounds. This property, coupled with the capability for asymmetric creation of chiral centers with chiral hydroboration agents, makes the reaction most valuable for asymmetric organic synthesis. In some of the cases, however, this has been achieved by diborane itself as shown in the synthesis of monensin by Kishi et al. A stereospecific synthesis of its seven carbon. component has been accomplished by two hydroboration reactions (Eq. 129) 209. ... 

Benzophenone synthase (BPS) and biphenyl synthase (BIS) catalyze the formation of the same linear tetraketide from benzoyl-CoA and three molecules of malonyl-CoA. However, BPS cyclizes this intermediate via intramolecular C6-+C1 Claisen condensation, whereas BIS uses intramolecular C2— C7 aldol condensation. Benzophenone derivatives include polyprenylated polycyclic compounds with hi pharmaceutical potential. Biphenyl derivatives are the phytoalexins of the economically important Maloideae. 

While rearrangement of propargylic esters to allenyl esters is facile, denouement of such intermediates is highly dependent on the presence of other multiple CC bonds in juxtaposi-tion. In any event, the generation of polycyclic compounds in one synthetic operation deserves serious consideration of the method for exploitation in the construction of significant and complex target molecules. 

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