Isomers hydroboration-oxidation

You can use either of the two methods you know to convert an alkyne into an alkene, because this alkene does not have cis-trans isomers. Hydroboration-oxidation must be used to convert the alkene into the desired alcohol, because acid-catalyzed addition of water would not form the desired alcohol. 

Though the triplet sensitized photolysis of isoprene (159) does, as noted above, produce a complex mixture of products, one of these adducts has been used in the context of complex molecule synthesis (equation 5)71. Cyclobutane 160, which was formed in ca 20% yield by the benzophenone sensitized photolysis of 159, could be easily transformed into fragrantolol, 161, an isomer of grandisol isolated from the roots of the Artemisia fragrans, by simple hydroboration/oxidation of the less hindered double bond. 

Diketo diester 512 has played a key role in one of Paquette s approaches to the pentagonal dodecahedrane. Direct hydroboration-oxidation of502 provided as the principal product the unwanted isomer 511 (49 %) rather than 512 (30 %). This complication was circumvented by the improvisation of the cross-corner oxygenation sequence outlined in Scheme 75.420,42 The iodolactonization of diacid 510 proceeded with high efficiency to give 513 which underwent cleavage to 514 in the presence of methanolic sodium methoxide at room temperature. Oxidation to a-iodo ketone 515 followed by reductive deiodination with zinc-copper couple and ammonium chloride in methanol solution furnished isomerically pure 512. 

Hydration (H2O, H" ) and hydroboration-oxidation (BH3 followed by H2O2, HO ) both add the elements of H2O across a double bond. Despite their similarities, these reactions often form different constitutional isomers, as shown in Sample Problem 10.6. 

A series of deprotection, reduction, and reprotection steps provided a 69% yield of silyl ether 226, which was subjected to a hydroboration-oxidation sequence to afford aldehyde 267 (87%) as a mixture of isomer at C-8. Aldehyde 267 was then converted to the Emmons reagent 268 via a sequence involving addition of lithium dimethyl methylphosphonate, oxidation to the keto phosphonate, deprotection, and oxidation in 65% overall yield. 

The Diels-Alder reaction was utilized to construct bicyclo [2.2 1]heptane or bicyclo[2 2 l]heptene structures The reaction of isopropylidenecyclopentadiene with maleic anhydride produced the endo and exo configurational isomers of 8-isopropylidenebicyclo[2.2.1] hept-2-ene-5,6-dicarboxylic anhydride Similar reactions were applied to unsubstituted and l-(methoxycarbonyl)cyclopentadienes to give the corresponding anhydrides The anhydrides were reduced to alcohols, which were then allowed to react with thionyl chloride or tosyl chloride to give cyclic sulfites or tosylates Reaction of the tosylates with lithium chloride gave chlorinated compounds Hydration of the double bonds of the chlorinated compounds was accomplished by hydroboration-oxidation Diol 31 thus obtained was converted to 5,6-bis(chloromethyl)-7-isopropylidene-bicyclo[2 2 1] heptan-2-one [33] by chromium trioxide oxidation of the secondary hydroxyl group followed by dehydration at the C-7 substituent. 

The Mitsunobu reaction has been used by It6 and co-workers as a key step in the synthesis of unnatural (+)-a-skytanthine (184) (196). The chiral amide 210 (197) was subjected to an aza-Claisen rearrangement, and the resulting amide 211 was reduced with LiAlU to afford the amine 212 in 80% yield. A hydroboration-oxidation sequence led to a mixture of amino alcohols 213, which was heated at 100°C for 24 h in the presence of cyanomethylene-trimethylphosphorane (CMMP) to yield a 92 8 mixture of cis- and rrans-fused isomers 214 and 215 in 81% yield from 212. 

Hydroboration-oxidation was used to hydrate the double bond of 156, resulting in the formation of 157 in 74% yield. Only minor amounts of the isomer with the hydroxyl at C-6 were formed. In straightforward steps 157 was converted into 148 and into the enone... 

Oxidation of the 17-acetal of estrone (29-1, from estrone and propylene-1,3-glycol) with DDQ abstracts hydrogen from ring C to form the styrenoid derivative 29-2 (Scheme 3.29). The driving force behind this particular regiochemistry may be related to the more transoid nature of that compound compared with the 6,7 isomer. Hydroboration of the product followed by oxidative workup affords the 1 la-hydroxy derivative 29-3. Oxidation of the newly introduced hydroxyl group with chromium trioxide followed by hydrolysis of the 17-acetal yields... 

Reaction of the ethyl thiolactam 501 with allyl bromide gave the salt 502 which on base-catalyzed rearrangement afforded 503 and a mixture of isomers. Hydroboration and alkaline oxidation of each isomer gave a mixture of alcohols (504 and 505) which were converted to quebrachamine (82) in the standard manner (Scheme 26). 

In this problem, 1-methylcyclopentene, an alkene, is the starting product. This must be converted into the alcohol so that it may be then transformed into the sodium alkoxide for use in the Williamson synthesis. (One does not want to convert 1-methylcyclo-pentene into an alkyl halide because a secondary or tertiary structure would result, and this is not very efficient, as mentioned previously.) Now, there exist a number of ways to synthesize an alcohol from an alkene. The method of choice, however, is the one that provides us with the required stereochemistry. Recall that the final product must be a trans isomer. If hydroboration-oxidation of the alkene is used, addition occurs in a cis fashion and the trans isomer will be the net result. That is, the -H and -OH add trans to the methyl group. 

The reaction in equation 9.52 does not provide a convincing demonstration of the stereochemistry of hydroboration-oxidation, since 52 is thermodynamically more stable than the corresponding cis isomer. Therefore, Brown and Zweifel also carried out the procedure on 1,2-dimethylcyclopentene (53, equation 9.53). The product of that reaction was the less stable stereoisomer... 

Our way to natural compounds assumes the use of enic hydrocarbon raw materials, which cause no problems in metathesis. Involving cycloolefms of various size C5-C9 with a-olefins to cometathesis we can prepare l,A-dienes of the desired length, internal bond position and Z-isomer content up to 79%. Their hydroboration-oxidation leads to many real insect sex pheromone components, alcohols, acetates, and aldehydes with double bond positions of 6-11. 

Devise an experimental procedure for hydroboration/oxidation of an alkene, the results of which will test the net anti-Markovnikov orientation of the overall reaction. One possible substrate would be 1-hexene. Be certain the spectral data are available in the literature for the possible isomers that could be formed, so that you can analyze the product(s) you obtain. Check with your instructor before undertaking the procedure. 

PROBLEM 15.74 2-Methyl-2 -butene can be hydrated in acid, or under hydroboration-oxidation conditions to give two different alcohols. Give structures for these alcohols and assign the following NMR spectra (1 and 2) to these isomers. Explain your answers. 

---