Asymmetric hydrogenation approaches to valuable, acyclic 1,3-hydroxymethyl chirons

Article abstract of DOI:10.1021/ja802909b

An iridium carbene oxazoline complex was used to catalyze hydrogenations of trisubstituted alkenes to give terminal and internal 1,3-hydroxymethyl chirons. The products are accessible in all possible stereoisomeric forms. These hydrogenations do not requi

Full text of DOI:10.1021/ja802909b

Published on Web 06/19/2008
Asymmetric Hydrogenation Approaches to Valuable, Acyclic
1,3-Hydroxymethyl Chirons
Ye Zhu and Kevin Burgess*
Department of Chemistry, Texas A & M UniVersity, Box 30012, College Station, Texas 77841
Received April 19, 2008; E-mail: burgess@tamu.edu
Research into directed hydrogenations for acyclic stereocontrol
peaked in the 1980s.1 Much of that work involved 1,1-disubstituted
alkenes that are sterically noncongested and easy to hydrogenate.
Examples involving trisubstituted alkenes are rarer and tend to
feature homoallylic alcohols.2 In these cases, high diastereoselec-
tivities are possible because simultaneous coordination of the metal
to the hydroxyl and alkene gives rigid, chairlike intermediates.
However, there are relatively few examples of useful acyclic
stereocontrol in hydrogenations of trisubstituted alkenes that are
allylic alcohols, or where no alcohol at all is present. There is a
good reason for this. Most of the work on directed hydrogenations
has featured rhodium and iridium catalysts of the type MP2 where
P2 is a chelating diphosphine ligand, and these do not hydrogenate
trisubstituted alkenes at a significant rate if a directing group is
not present. This is not so, however, for Crabtree’s catalyst3–6 and
analogues of the IrN,P type (N,P ) sp2-N ligand and phosphine);
they can hydrogenate trisubstituted and even tetrasubstituted alkenes
where there are no apparent directing groups.7 Few investigations
of acyclic stereocontrol, however, have focused on Crabtree’s
catalyst,8,9 and in those, the stereoselectivities obtained were poor.
No studies on acyclic stereocontrol have featured optically active
analogues of this catalyst; indeed, the first of these complexes was
not reported until 1998.10 This is a significant gap in the literature
because chiral catalysts can constructively couple with substrate
biases (matching effects),11 and these catalysts, unlike diphosphine
systems for the same substrates, can exert strong influences.12–14
This communication describes applications of these concepts in
hydrogenations mediated by complex 1 to afford the “internal” and
“terminal” chirons A and B found in many natural products and
useful derivatives.
Figure 1. Preparation of (a) a syn-type A chiron, and (b) an anti-type A
chiron. All ratios quoted are from GC. (c) Directed hydrogenation model
gives substrate control, and (d) catalyst control dominates where the substrate
conformation is only held by 1,3-allylic strain.
be used to rationalize preferential formation of the product 3, so
this is a typical directed hydrogenation of a homoallylic alcohol
via substrate control (Figure 1c). Formation of 5, however, is
catalyst-controlled. In that case, there is a substrate vector based
on conformers preferred by 1,3-allylic strain considerations and
possibly some directing effect from the allylic alcohols; however,
these two factors modulate, but do not overcome, the catalyst vector.
A similar approach was used to obtain optically pure syn- and
anti-isomers corresponding to the terminal fragment B, except that
lactic acid was the starting material. Seven relevant substrates were
prepared and tested, but the discussion here is limited to hydrogena-
tion of the two that gave the highest syn and anti selectivities, that
is, 6 and 8 (Figure 2). Catalyst control dominated for substrate 6,
and the optimal stereoselectivity was obtained when the 1,3-allylic
strain vector from the substrate matched with the preferred approach
of the catalyst (Figure 2c). Hydrogenation of 8 was substrate
controlled. In this example, the alkene is quite hindered. The
simplest explanation for the observed selectivity is in terms of
directed attack resulting from coordination to the allylic alcohol.
This could occur via oxygen coordinating with iridium or via
hydrogen bonding from an iridium hydride to the allylic alcohol
oxygen.16,17 The latter is possible since recent observations from
our group indicate the iridium complex is slightly acidic.18
Syntheses of substrates to prepare the internal fragments A began
with glycidol acetonide (conveniently available as either enanti-
omer).15 Hence compounds 2, 4, and some other esters and allylic
alcohols were prepared. Several of these were hydrogenated; only
the best data are shown here. The substrate that corresponds to 2
but with the alcohol and silyl ether groups juxtaposed did not give
high selectivities (not shown), but excellent data were obtained for
2 and 4 (Figure 1).
Formation of the syn product 3 is an excellent reaction because
this lactone is crystalline and it can be recrystallized from the crude
material without column chromatography. In both reactions, the
diastereoselectivity is extremely high. A chairlike intermediate can
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8894 J. AM. CHEM. SOC. 2008, 130, 8894–8895
10.1021/ja802909b CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
build terminal 1,3-hydroxymethyl chirons. Thus the allylic alcohol
10 was prepared from commercially available (R)-2-hydroxy-4-
methylpentanoic acid C via a series of standard steps (see
Supporting Information). Hydrogenation of this under our standard
conditions gave the crude alcohol in excellent diastereoselectivity.
The iso-butyl-for-methyl substitution that connects Figure 2c with
the hydrogenation in Scheme 1 had no adverse effect on the
stereoselectivity. After flash chromatography, the desired product
was isolated in high yield, and the diastereomeric impurity was
hardly perceptible by GC. Formation of the iodide 12, homologation
with sulfoxide 13, reduction, then deprotection afforded the product
14.
This communication attempts to convey several key points. First,
constructive matching of chiral Crabtree’s catalyst analogues with
stereochemical vectors from substrates can afford high diastereo-
selectivities, even in cases where Ir- or Rh-diphosphine complexes
would probably give poor conversions and/or selectivities. We have
previously observed catalyst control dominating hydrogenation of
some substrates leading to deoxypolyketides.12–14 The fact that this
is not uniformly so here enhances the scope of the approach;
mechanistic complementarities enabled all stereoisomers of the
ubiquitous chiral fragments A and B to be made. Reactions that
gave lesser selectivities but led to the development of these highly
stereoselective processes will be described in a full account of this
work, along with other applications.
Acknowledgment. We acknowledge Dr. Jianguang Zhou for
some preliminary experiments. Financial support for this work was
provided by The National Science Foundation (CHE-0750193) and
The Robert A. Welch Foundation.
Figure 2. Preparation of type B chirons: (a) a syn-form, and (b) an anti-
form. All ratios quoted are from GC. (c) Catalyst control dominates where
the substrate conformation is only held by 1,3-allylic strain, and (d) substrate
control prevails for substrate 8.
Supporting Information Available: Experimental procedures for
the preparation of compounds 2 to 14, details of the determination of
enantiomeric excesses, and assignments of absolute configurations. This
material is available free of charge via the Internet at http://pubs.acs.org.
Scheme 1. Total Synthesis of (-)-Dihydromyoporone
References
(1) Brown, J. M. Angew. Chem., Int. Ed. Engl. 1987, 26, 190–203.
(2) Evans, D. A.; Morrissey, M. M.; Dow, R. L. Tetrahedron Lett. 1985, 26,
6005–6008.
(3) Crabtree, R. H.; Felkin, H.; Morris, G. E. Chem. Commun. 1976, 716–
717.
(4) Crabtree, R. H.; Felkin, H.; Morris, G. E. J. Organomet. Chem. 1977, 141,
205–215.
(5) Crabtree, R. H.; Morris, G. E. J. Organomet. Chem. 1977, 135, 395–403.
(6) Crabtree, R. H. Acc. Chem. Res. 1979, 12, 331–337.
(7) Cui, X.; Burgess, K. Chem. ReV. 2005, 105, 3272–3296.
(8) Evans, D. A.; Morrissey, M. M. J. Am. Chem. Soc. 1984, 106, 3866–3868.
(9) Evans, D. A.; Morrissey, M. M. Tetrahedron Lett. 1984, 25, 4637–4640.
(10) Lightfoot, A.; Schnider, P.; Pfaltz, A. Angew. Chem., Int. Ed. 1998, 37,
2897–2899.
(11) Masamune, S.; Choy, W.; Peterson, J. S.; Sita, L. R. Angew. Chem., Int.
Ed. Engl. 1985, 24, 1–30.
(12) Zhou, J.; Burgess, K. Org. Lett. 2007, 9, 1391–1393.
(13) Zhou, J.; Ogle, J. W.; Fan, Y.; Banphavichit, V.; Zhu, Y.; Burgess, K.
Chem.—Eur. J. 2007, 13, 7162–7170.
(14) Zhou, J.; Burgess, K. Angew. Chem., Int. Ed. 2007, 46, 1129–1131.
(15) Hanson, R. M. Chem. ReV. 1991, 91, 437.
(16) Crabtree, R. H. Science 1998, 282, 2000–2001.
(17) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,
4th ed.; Wiley: New York, 2005.
(18) Zhu, Y.; Burgess, K. AdV. Synth. Catal. 2008, 350, 979–983.
(19) Johnson, W. S.; Crackett, P. H.; Elliott, J. D.; Jagodzinski, J. J.; Lindell,
S. D.; Natarajan, S. Tetrahedron Lett. 1984, 25, 3951–3954.
A synthesis of (-)-dihydromyoporone 1419,20 was performed
to illustrate that R-hydroxyacids other than lactic could be used to
(20) Still, W. C.; Darst, K. P. J. Am. Chem. Soc. 1980, 102, 7385–7387.
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