A-selenation

In an effort to make productive use of the undesired C-13 epimer, 100-/ , a process was developed to convert it into the desired isomer 100. To this end, reaction of the lactone enolate derived from 100-) with phenylselenenyl bromide produces an a-selenated lactone which can subsequently be converted to a,) -unsaturated lactone 148 through oxidative syn elimination (91 % overall yield). Interestingly, when 148 is treated sequentially with lithium bis(trimethylsilyl)amide and methanol, the double bond of the unsaturated lactone is shifted, the lactone ring is cleaved, and ) ,y-unsaturated methyl ester alcohol 149 is formed in 94% yield. In light of the constitution of compound 149, we were hopeful that a hydroxyl-directed hydrogenation52 of the trisubstituted double bond might proceed diastereoselectively in the desired direction In the event, however, hydrogenation of 149 in the presence of [Ir(COD)(py)P(Cy)3](PF6)53 produces an equimolar mixture of C-13 epimers in 80 % yield. Sequential methyl ester saponification and lactonization reactions then furnish a separable 1 1 mixture of lactones 100 and 100-) (72% overall yield from 149). 

Two mechanisms have been suggested for the reaction with Se02. One of these involves a selenate ester of the enol ... 

A Selen, PG Welling. Bioavailability of hydrochlorothiazide from tablets and suspensions. J Pharm 18. Sci 73 359-365, 1984. 

In a similar way, a-nitroselenides are prepared via the reaction of nitronates with phenylse-lenyl bromide, which gives a new synthetic method of l-nitroalkenes from nitroalkanes.130 The sequence of a-selenation, nitro-aldol reaction, and oxidation provides a useful method for the preparation of nitroalkenes with a hydroxymethyl group (Eq. 5.81).131... 

The diol function of 130 was protected as its acetonide 131 (88 %). Next, the enone function was installed by a-selenation of the enoxysilane, followed by peroxide oxidation and elimination (57 % over two steps). Finally, the unsaturated ketone 132 was homologated by 1,4-addition of trimethylsilylmethyl magnesium chloride, trapping with chlorotrimethylsilane, and reoxidation, to afford the target 117 (88 %). 

Oxyselenation is a powerful tool for functionalization in synthetic organic chemistry. Electrochemically generated [Br]+ or bromine reacts with (PhSe)2, producing PhSeBr. Attack of PhSeBr on the olefin (25) gives a selenated intermediate followed by solvolysis to provide (26) and a bromide ion (Scheme 11). A typical electrolysis is carried out in a MeOH-Et4NBr/H2S04 (trace)/(PhSe)2-(Pt) system in an undivided cell. Some results... 

Problem 3.30 Why is a sulfonic acid, such as methanesulfonic acid (CH,SO,H), a stronger acid than a selenic acid, such as methaneselenic acid (CH,SeO,H) ... 

Wang and co-workers recently developed an efficient strategy for the a-selenation of aldehydes and ketones using the proline derivatives as catalysts under mild conditions, the products were obtained in good yield (Eq. 10) [35]. 

To use a selenate instead of a sulfoxide rearrangement for the preparation of allylic alcohols can be advantageous if silicon is present in the starting material. Vedejs found that the allylsilane (213) could be selenylated to give (214 equation 79) this reaction includes a rapid 1,3-shift of the initially formed selenide. Oxidation and 2,3-rearrangement to the alcohol (215) followed uneventfully. 

The enolate may attack the selenium atom of the selenium dioxide to form a selenate ester of the enol. This intermediate may rearrange to re-form the carbonyl group, and transfer an oxygen atom to the a-carbon. Suggest how this may occur. 

D.25 (a) selenic acid (b) sodium arsenate (c) calcium tellurite (d)... 

Selenium trioxide melts at (392.5 1 1.5) K, [62JER/MEN], [63MIJ/GER], to a colourless liquid. The decomposition of SeO ,(l) on heating was studied by Jerschkewitz and Menning [62JER/MEN] and found to be dependent on the purity of the specimen. The presence of a selenate salt, selenium dioxide, or water greatly enhances the rate of the decomposition reaction ... 

Selivanova, Zubova, and Finkel stein [59SEL/ZUB] also made a calorimetric study of the reaction between a silver nitrate and a selenic acid solution with formation of Ag2Se04(cr). The standard enthalpy of formation of the compound has been evaluated from the data in Appendix A. The value obtained there ... 

Thorium forms a silicide, ThSi2, two borides,2 ThB4 and ThB6, a selenate, Th(Se04)2 9 II20, and a selenite, Th(Se03)2 H20, besides numerous salts of organic acids. 

Compound 68 was subjected to another annelation with MVK to give the tricyclic s/nro-ketone 21. It was also transformed to. sp/ro-dienone 72 via a selenation-elimination process. 

Maenhaut, W., H. Raemdonck, A. Selen, R. Van Grieken, and J. W. Winchester (1983). Characterization of the atmospheric aerosol over the eastern equatorial Pacific. J. Geophys. Res. 88, 5353-5364. 

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