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L. K. Blasdel et al. / Bioorg. Med. Chem. Lett. 23 (2013) 6905–6910
To transform the aldol addition product 4 into enantiomerically
pure dihydrosalinosporamide A (2), we developed the 7-step
sequence shown in Scheme 1. The sequence began with transeste-
rification of 4 in the presence of 2-trimethylsilylethanol and titanium
isopropoxide9 (recovery of the volatile chiral alcohol by-product
was not attempted) affording the 2-trimethylsilylethyl ester 5
in 93% yield. Alkylative introduction of the C2 substitutent
was then achieved in one operation by in situ N-tert-butyldimeth-
ylsilylation (PhLi; TBSCl) followed by formation of an extended
enolate at –78 °C with LDA and trapping with 2-chloroethyl tri-
flate. The alkylation reaction was position- and stereo-specific, pro-
viding exclusively the a-adduct 6. Stereoselective epoxidation then
occurred in the presence of trifluoroperacetic acid to provide the
epoxide 7 in 65% yield (two steps, from 5). Epoxide opening with
MgI210 gave rise to the iodohydrin 8. Exposure of the latter product
to triflic acid led to cleavage of the 2-(trimethylsilyl)ethyl ester
function as well as the tert-butyldimethylsilyl ether; selective
deiodination of the resulting iodo acid then occurred in the pres-
ence of Raney nickel.11 Lastly, the b-lactone ring was formed in
the presence of BOP-Cl and triethylamine,4 completing the synthe-
sis of dihydrosalinosporamide A (2, 7 steps from 4, 38% yield).
Salinosporamide analogs with 5-cyclopropyl and 5-benzyl
substituents in place of cyclohexyl were synthesized using cyclo-
propanecarboxaldehyde and 2-(cyclohexa-2,5-dien-1-yl)acetalde-
hyde,12 respectively, as alternative substrates in the Mukaiyama
aldol coupling with the pyrrole ester substrate 3 (Scheme 2).
Although the yields of the diastereomerically pure anti-aldol
products were only modest (due to need for extensive purification),
the transformations nevertheless provided sufficient material to
permit further processing of these products by sequences analo-
gous to Scheme 1 to produce the corresponding salinosporamide
analogs (compounds 17 and 18, respectively, Table 2) for
evaluation of proteasomal inhibition (8–9 steps, 6–10% yield, see
Supplementary data for details).
Scheme 1. Synthesis of dihydrosalinosporamide A (2).
mixtures, although differing proportions of the three alternative
diastereomeric addition products were also evident (Table 1).
While we believe that in each case the major product was likely
an anti diastereomer, we did not rigorously establish this, but
instead focused on identifying the most useful substrate in terms
of the net diastereoselectivity of the aldol addition reaction and
the ease of purification of the major product. This proved to be
the 4-trimethylsilyl-3-butyn-2-ol ester substrate of entry 10. The
parent chiral alcohol used to prepare this substrate is readily avail-
able in gram amounts in both enantiomeric forms by asymmetric
hydrogenation of 4-trimethylsilyl-3-butyn-2-one using the Noyori
protocol.8
In one larger-scale implementation of the transformation of
entry 10 (Table 1), we added tert-butyldimethylsilyl trifluorometh-
anesulfonate (9.6 mL, 42 mmol, 2.0 equiv) to a solution of the
pyrrole ester substrate 3 (7.9 g, 21 mmol, 1 equiv) and cyclohex-
anecarboxaldehyde (3.8 mL, 31 mmol, 1.5 equiv) in dichlorometh-
ane (104 mL) at À78 °C. After 6 h, triethylamine was added, and
the product mixture was isolated by extraction. Diastereomerically
pure anti aldol addition product 4 (4.4 g, 43% yield) was obtained
by flash-column chromatography, then trituration with hexanes.
X-ray crystallographic analysis (Fig. 2) established that the product
we obtained was the (4S,5S)-anti stereoisomer.
In addition to variation of the aldehyde coupling partner, we
employed alternative electrophiles for enolate trapping and so pro-
duced analogs with different C2-substituents. For example, use of
3-chloropropyl triflate as the electrophile provided access to the
homologated dihydrosalinosporamide analog 16 (Scheme 3 and
Table 2) whereas allyl bromide provided access to the aldehyde
and ester analogs 19 and 20 by selective oxidation of the allyl
side-chain (11 ? 12, Scheme 4). The latter
a-allylation reaction
employed a different protocol for in situ N-tert-butyldimethylsily-
lation, using N-tert-butyldimethylsilyl-N-methyltrifluoroaceta-
mide, which allowed us to isolate and vacuum-dry the N-protected
lactam prior to adding LDA. All three analogs (16, 19, and 20, see
Table 2) were prepared with the goal of exploring alternative
modes of cyclization of the hydroxyl group that is liberated upon
opening of the b-lactone function by the N-terminal threonine
residue of the chymotryptic subunit of the proteasome (see
Fig. 3), which has been suggested to be an important factor in
the functioning of salinosporamide A.2b,6e,13
To evaluate the ability of each salinosporamide analog to inhibit
the chymotryptic site of the 20S proteasome, an in vitro assay was
conducted using purified human 20S proteasome and a commer-
cial fluorogenic substrate14 (assays conducted by Dr. Sridevi Pond-
uru and Mr. Ronald Paranal, laboratory of Prof. James Bradner,
Dana Farber Cancer Institute). The IC50 values that were obtained
are listed in Table 2. Dihydrosalinosporamide A was found to be
somewhat less potent (one to threefold) than an authentic sample
of salinosporamide A (in a prior study, dihydrosalinosporamide A
(2) was found to be ꢀeightfold less potent than 1 in an assay of
the chymotryptic activity of rabbit 20S proteasome).2b Interest-
ingly, we found that the 5-cyclopropyl analog 17 and 1 displayed
similar potencies while the 2-(3-chloropropyl) analog 16 was
two to threefold less potent than 1.15 In contrast, the 5-benzyl
Figure 1. A diastereoselective Mukaiyama aldol reaction featured in
a prior
synthesis of lactacystin,7 numbered to correspond with salinosporamide A.
Blasdel, Landy K.
