Enantiodivergent conversion of chiral secondary alcohols into tertiary alcohols

Enantioselective synthesis of (R)-tolterodine using lithiation/borylation–protodeboronation methodology

The synthesis of the pharmaceutical (R)-tolterodine is reported using lithiation/borylation–protodeboronation of a homoallyl carbamate as the key step. This step was tested with two permutations: an electron-neutral aryl Li-carbamate reacting with an electron-rich boronic ester and an electron-rich aryl Li-carbamate reacting with an electron-neutral boronic ester. It was found that the latter arrangement was considerably better than the former. Further improvements were achieved using magnesium bromide in methanol leading to a process that gave high yield and high enantioselectivity in the lithiation/borylation reaction. The key step was used in an efficient synthesis of (R)-tolterodine in a total of eight steps in a 30% overall yield and 90% ee.

Synthetic applications and inversion dynamics of configurationally stable 2-lithio-2-arylpyrrolidines and -piperidines

In diethyl ether, N-Boc-2-lithio-2-arylpiperidines have been found to be configurationally stable at -80 °C, whereas N-Boc-2-lithio-2-arylpyrrolidines are configurationally stable at -60 °C. Several tertiary benzylic carbanions derived from enantioenriched 2-aryl heterocycles have been successfully alkylated or acylated with little to no loss of enantiopurity. The scope of the reactions has been explored. The enantiomerization dynamics of N-Boc-2-lithio-2-phenylpyrrolidine and N-Boc-2-lithio-2-phenylpiperidine have been studied in the presence of different solvents and achiral ligands.

Nickel-catalyzed coupling reactions of alkyl electrophiles, including unactivated tertiary halides, to generate carbon-boron bonds

Through the use of a catalyst formed in situ from NiBr(2)·diglyme and a pybox ligand (both of which are commercially available), we have achieved our first examples of coupling reactions of unactivated tertiary alkyl electrophiles, as well as our first success with nickel-catalyzed couplings that generate bonds other than C-C bonds. Specifically, we have determined that this catalyst accomplishes Miyaura-type borylations of unactivated tertiary, secondary, and primary alkyl halides with diboron reagents to furnish alkylboronates, a family of compounds with substantial (and expanding) utility, under mild conditions; indeed, the umpolung borylation of a tertiary alkyl bromide can be achieved at a temperature as low as -10 °C. The method exhibits good functional-group compatibility and is regiospecific, both of which can be issues with traditional approaches to the synthesis of alkylboronates. In contrast to seemingly related nickel-catalyzed C-C bond-forming processes, tertiary halides are more reactive than secondary or primary halides in this nickel-catalyzed C-B bond-forming reaction; this divergence is particularly noteworthy in view of the likelihood that both transformations follow an inner-sphere electron-transfer pathway for oxidative addition.

An experimental and in situ IR spectroscopic study of the lithiation-substitution of N-Boc-2-phenylpyrrolidine and -piperidine: controlling the formation of quaternary stereocenters

A general and enantioselective synthesis of 2-substituted 2-phenylpyrrolidines and -piperidines, an important class of pharmaceutically relevant compounds that contain a quaternary stereocenter, has been developed. The approach involves lithiation-substitution of enantioenriched N-Boc-2-phenylpyrrolidine or -piperidine (prepared by asymmetric Negishi arylation or catalytic asymmetric reduction, respectively). The combined use of synthetic experiments and in situ IR spectroscopic monitoring allowed optimum lithiation conditions to be identified: n-BuLi in THF at -50 °C for 5-30 min. Monitoring of the lithiation using in situ IR spectroscopy indicated that the rotation of the tert-butoxycarbonyl (Boc) group is slower in a 2-lithiated pyrrolidine than a 2-lithiated piperidine; low yields for the lithiation-substitution of N-Boc-2-phenylpyrrolidine at -78 °C can be ascribed to this slow rotation. For N-Boc-2-phenylpyrrolidine and -piperidine, the barriers to rotation of the Boc group were determined using density functional theory calculations and variable-temperature (1)H NMR spectroscopy. For the pyrrolidine, the half-life (t(1/2)) for rotation of the Boc group was found to be ∼10 h at -78 °C and ∼3.5 min at -50 °C. In contrast, for the piperidine, t(1/2) was determined to be ∼4 s at -78 °C.

Tertiary alcohol preferred: Hydroxylation of trans-3-methyl-L-proline with proline hydroxylases

The enzymatic synthesis of tertiary alcohols by the stereospecific oxidation of tertiary alkyl centers is a most-straightforward but challenging approach, since these positions are sterically hindered. In contrast to P450-monooxygenases, there is little known about the potential of non-heme iron(II) oxygenases to catalyze such reactions. We have studied the hydroxylation of trans-3-methyl-L-proline with the α-ketoglutarate (α-KG) dependent oxygenases, cis-3-proline hydroxylase type II and cis-4-proline hydroxylase (cis-P3H_II and cis-P4H). With cis-P3H_II, the tertiary alcohol product (3R)-3-hydroxy-3-methyl-L-proline was obtained exclusively but in reduced yield (~7%) compared to the native substrate L-proline. For cis-P4H, a complete shift in regioselectivity from C-4 to C-3 was observed so that the same product as with cis-P3H_II was obtained. Moreover, the yields were at least as good as in control reactions with L-proline (~110% relative yield). This result demonstrates a remarkable potential of non-heme iron(II) oxygenases to oxidize substrates selectively at sterically hindered positions.

Catalytic enantioselective 1,2-diboration of 1,3-dienes: versatile reagents for stereoselective allylation

More with boron: The development of catalytic enantioselective 1,2-diboration of 1,3-dienes enables a new strategy for enantioselective carbonyl allylation reactions (see scheme). These reactions occur with outstanding levels of stereoselection and can be applied to both monosubstituted and 1,1-disubstituted dienes. The carbonyl allylation reactions provide enantiomerically enriched functionalized homoallylic alcohol products.

Biocatalytic synthesis of optically active tertiary alcohols

The enzymatic preparation of optically pure tertiary alcohols under sustainable conditions has received much attention. The conventional chemical synthesis of these valuable building blocks is still hampered by the use of harmful reagents such as heavy metal catalysts. Successful examples in biocatalysis used esterases, lipases, epoxide hydrolases, halohydrin dehalogenases, thiamine diphosphate-dependent enzymes, terpene cyclases, -acetylases, and -dehydratases. This mini-review provides an overview on recent developments in the discovery of new enzymes, their functional improvement by protein engineering, the design of chemoenzymatic routes leading to tertiary alcohols, and the discovery of entirely new biotransformations.

Retention of regiochemistry and chirality in the ruthenium catalyzed allylic alkylation of disubstituted allylic esters

The regiospecific nucleophilic substitution during the ruthenium catalyzed allylic alkylation of 1,3-unsymmetrical disubstituted allylic esters was demonstrated. The nucleophile was selectively introduced at the position originally substituted with leaving group in the 2-DPPBA or ip-pybox ligated [RuCl(2)(p-cymene)](2) catalyzed allylic alkylation of 1,3-unsymmetrical disubstituted allylic esters. The chirality of the optically active allylic esters was also transferred to the alkylated products.