Ketones synthesis from organoboranes

Organoboranes are used for ketone synthesis under basic conditions. The cyclic ketone 482 is prepared from alkenyl iodode 479. Hydroboration of terminal double bond, followed by carbonylation generates 480, and the cyclic ketone 482 is formed by intramolecular transmetallation of 480 to afford 481 [236],... 

Synthesis of pure enantiomers. Brown and Singaram1 have reviewed the chiral organoboranes obtained from (+)- and (- )-pinene and their use for synthesis of optically pure amines, halides, hydrocarbons, lithium alkylborohydrides, ketones, aldehydes, a-chiral acids, esters, nitriles, alkynes, and alkenes. 

The reaction of organoboranes with catechol dichloromethylene ether in the presence of methyllithium provides a synthesis of symmetrical ketones (Eq. 34) From the result of a crossover experiment by using a mixture of two different trialkylboranes, the reaction was confirmed to proceed through intramolecular reaction pathways. 

However, this methodology to homologate a-bromoacetone for the synthesis of methylketones fails. This ketone is extraordinarily sensitive to the action of potassium t-butoxide, an butoxide, an exceptionally strong base. The problem is circumvented by utilizing potassium 2,6-di-t-butylphenoxide. The presence of bulky substituents at ortho positions prevent the organoborane from coordinating with base. 

Stereoselective addition of B-Br across a terminal acetylene (haloboration) was first reported by Blackborow (66). Suzuki and coworkers examined the intermediate obtained from the haloboration of alkynes with B-bromo and 5-iodo-9-borabicyclo[3.3.1]nonanes for various organoborane reactions. Suzuki has reviewed the applications of haloboration for organic syntheses (67-69). A recent application involving the 1,4-addition of a halovinyl-9-BBN to methyl vinyl ketone (70) for the synthesis of a promising anti-cancer agent, 12,13-desoxyepothilone B due to Danishefsky and coworkers is shown in Figure 19 (77). 

A review of the current status of the applications of chiral organoboranes derived from a-pinene for the synthesis of fluoroorganic molecules has been made. Our three-pronged approach for the synthesis of asymmetric fluoro-organics involve (i) asymmetric reduction of fluoiinated ketones, (ii) asymmetric allylboration of fluoiinated aldehydes, and (iii) asymmetric enolboration-aldolization of fiuoro-ketones and aldehydes. It appears that the presem e of fluorite atom(s) in the molecule influences the stereoctemi< outcome in asymmetric reduction and asymmetric enolboration-aldolization using cMorodiisopinocampheylborane. 

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