Phosphazene-catalyzed desymmetrization of cyclohexadienones by dithiane addition

Article abstract of DOI:10.3762/bjoc.13.75

We report a desymmetrization of cyclohexadienones by intramolecular conjugate addition of a tethered dithiane nucleophile. Mild reaction conditions allow the formation of diversely functionalized fused bicyclic lactones. The products participate in facially selective additions from the convex surface, leading to allylic alcohol derivatives.

Full text of DOI:10.3762/bjoc.13.75

Phosphazene-catalyzed desymmetrization of
cyclohexadienones by dithiane addition
Matthew A. Horwitz‡, Elisabetta Massolo‡ and Jeffrey S. Johnson*
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 762–767.
Department of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, NC 27599-3290, USA
doi:10.3762/bjoc.13.75
Received: 21 January 2017
Accepted: 04 April 2017
Published: 24 April 2017
Email:
Jeffrey S. Johnson* - jsj@unc.edu
* Corresponding author ‡ Equal contributors
Associate Editor: T. P. Yoon
Keywords:
© 2017 Horwitz et al.; licensee Beilstein-Institut.
License and terms: see end of document.
conjugate addition; cyclohexadienones; dearomatization;
desymmetrization; dithiane; iminophosphoranes; organocatalysis;
phosphazenes
Abstract
We report a desymmetrization of cyclohexadienones by intramolecular conjugate addition of a tethered dithiane nucleophile. Mild
reaction conditions allow the formation of diversely functionalized fused bicyclic lactones. The products participate in facially
selective additions from the convex surface, leading to allylic alcohol derivatives.
Findings
Desymmetrization has become a well-developed strategy for the and bisphenylsulfonyl [15] nucleophiles using bifunctional
construction of complex molecular frameworks [1-6]. Cyclo- cinchona alkaloid catalysts. The Sasai and Enders groups used
hexadienones are multipurpose synthetic building blocks that a phosphinothiourea to enable a Rauhut–Currier reaction to
have found a central role in desymmetrization methodologies. form bicyclic enones [16]. The Tian and Lin group used
The functional groups present in these symmetrical molecules alkyne-tethered cyclohexadienones in an arylrhodation/conju-
allow for a wide array of downstream transformations and they gate addition sequence that enantioselectively delivered oxabi-
are all formed through a single reaction from cheap and readily cyclo[4.3.0]nonanes [17]; the Lautens and Lan groups have also
available aromatic feedstocks [7-11]. These substrates have contributed to the further development of this reaction [18,19].
been successfully employed in a number of stereoselective The Rovis group employed cyclohexadienone hydroperoxides
desymmetrization reaction manifolds. Intramolecular Michael in a chiral phosphoric acid-catalyzed [1,2]/[1,4]-addition
additions via enamine intermediates have been studied by the cascade [20]. The same group also developed an acyl anion ad-
Gaunt [12] and Johnson groups [13]. The You group has dition promoted by N-heterocyclic carbenes (NHC) that
disclosed methods for the intramolecular addition of amine [14] furnished bicyclic furanones via Stetter addition [21]; later, the
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Beilstein J. Org. Chem. 2017, 13, 762–767.
You group developed an extension of this theme using the same Based on a prior report [30] demonstrating the efficacy of phos-
catalytic manifold [22]. More recently, the Corey group has phazene bases in deprotonating carboxylate dithianes, we
enabled the enantioselective conjugate reduction of prochiral selected the commercially available achiral superbase P2-t-Bu
cyclohexadienones using copper hydride generated in situ [23]. phosphazene to initiate the ring closure (Scheme 2) [31,32]. We
Inspired by these advances, we sought to develop an alternative found that in the simplest case, with the methyl-substituted
and complementary method invoking the dithiane moiety as an para-quinol ester (1a), the reaction was complete in 30 min at
established and easily accessible glyoxylate anion surrogate ambient temperature with 20 mol % catalyst [33]. Extending the
[24-29]. This would in principle provide access to highly func- length of the alkyl chain, the reaction proceeded similarly, even
tionalized products with orthogonally protected carbonyl groups in the presence of a methyl ester or a TBS-protected primary
in a novel glycolic acid scaffold.
alcohol (1b–d); a comparable result was observed with a phenyl
substituent (1e). We considered that if a nucleophilic group
We envisioned utilizing para-quinol derivatives featuring a were appended to the para-quinol, it would be possible to
tethered nucleophile as desymmetrization substrates, with the construct a 5–6–5 fused ring system. Indeed, when R =
intention of implementing a Brønsted base organocatalyzed ad- CH2CH2NHBoc (1f), the desired tricyclic product 2f was ob-
dition (Scheme 1). This reaction would lead to bicyclic systems tained. In all cases only a single diastereomer was observed. In
with the salient attribute of having a convex-concave facial dif- a substrate where a β-methyl group is present on the cyclohexa-
ferentiation, allowing subsequent diastereoselective transfor- dienone (1g), the reaction proved to be completely regioselec-
mations. With the aim of using a dithiane nucleophile, we tive, only allowing conjugate addition to the less substituted po-
selected 1,3-dithiane-2-carboxylic acid because of its relatively sition.
low pKa (compared with non-carboxylate substituted analogs)
and the possibility of using an ester linkage as a tether. We We attempted to render the reaction enantioselective using
found that the heretofore unknown dicyclohexylcarbodiimide chiral iminophosphoranes (Figure 1) structurally related to
(DCC) mediated coupling between para-quinols and 1,3- P2-t-Bu phosphazene, which are known to be substantially
dithiane-2-carboxylic acid proceeds in a straightforward manner more basic than trialkylamines [34]. With C1 [34-38] and C2
in cases where R is unbranched (though it does work for [39-55], we observed no product formation, presumably due to
R = Ph). Using this method, we were able to easily generate insufficient basicity. Though C3 [30,56,57] led to partial
diversely functionalized dithiane-linked para-quinols to study conversion of starting material, no appreciable enantioselectivi-
the intramolecular cyclization.
ty was observed.
To investigate the feasibility of a convex-facial addition, we
subjected 2a to Luche reduction conditions (Scheme 3). We
found this transformation to be completely diastereoselective,
and an X-ray diffraction study [58] of the product confirmed
our hypothesis regarding the facial selectivity, as the hydride
was delivered to the convex face. An analogous reaction occurs
when 2a is treated with AlMe3, affording the 1,2-addition prod-
uct (Scheme 3) [59-63].
Scheme 1: Desymmetrization of cyclohexadienone by tethered
nucleophile.
Figure 1: Chiral iminophosphorane catalysts surveyed.
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Beilstein J. Org. Chem. 2017, 13, 762–767.
Scheme 2: Scope of the transformation.
We next sought to establish the glyoxylate anion equivalency of We attempted to synthesize 5 via an alternative route using
the dithiane substructure in our system. In order to reveal the Cu(II)-catalyzed aerobic oxidative deacylation [76] of the
masked carbonyl functionality, we rigorously applied reported β-keto ester 6 (Scheme 4) [77]. The fact that this reaction also
dithiane deprotection conditions to 2a (Table 1). Despite exten- leads to decomposition of the starting material is cause for
sive investigations, none of our efforts were fruitful, resulting in general concern about the feasibility of easily reaching the
either no conversion, side reactions [64], or decomposition. We target substructure. In order to minimize the observed side reac-
rationalized these disappointing results considering: 1) the tions, we sought to apply the deprotection conditions to allylic
crowded steric environment surrounding the dithiane moiety on alcohol 3. However, both the use of NBS and HgCl2/HgO were
the concave face of the bicycle, 2) the sensitive nature of this unsuccessful.
class of compounds, which stems from the highly reactive func-
tional groups present, and 3) the strained character of the five- We further investigated the removal of the dithiane moiety via
membered α-ketolactone (5) that would result from deprotec- Raney nickel-promoted desulfurization (Scheme 5). To observe
tion.
any substrate conversion, it was necessary to use a hydrogen at-
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Beilstein J. Org. Chem. 2017, 13, 762–767.
Scheme 3: Convex facial additions.
Table 1: Carbonyl deprotection conditions.
Entry
Conditions
Result
1 [65]
2 [66]
3 [67]
4 [68]
5 [69]
6 [70]
7 [71]
8 [72]
9 [73]
10 [74]
11 [75]
NBS, MeCN/H2O, 0 °C, 10 mina
side reaction
side reaction
no reactionc
side reaction
no reaction
NBS, AgNO3, MeCN/H2O, 0 °C, 5 minb
PhI(OAc)2, MeCN/CH2Cl2/H2O, 50 °C, 18 h
Hg(ClO4)2, MeOH/CH2Cl2, rt, 2 h
HgCl2,HgO, MeOH/H2O, 55 °C, 18 hd
MeI, MeCN/H2O, reflux, 18 h
no reaction
m-CPBAe, MeCN, rt, 18 h, then 1 M HCl, reflux, 4 h
SbCl5, CH2Cl2, 0 °C, 1 h
decomposition
decomposition
no reaction
I2, NaHCO3, acetone/H2O, 50 °C, 18 h
CANf, acetone/H2O, 50 °C, 18 h
no reaction
Chloramine T, MeOH/H2O, 70 °C, 18 h
no reaction
aDifferent solvent systems, such as acetone/H2O and DMSO were used, stoichiometry was varied and the reaction was run also at rt and for longer
times (4 and 18 h) but in none of the cases was the desired product obtained. bThe reaction was also run at rt for 18 h, but the desired product was
not obtained. cDecomposition products were also observed. dThe MeCN/H2O solvent system was also used and the reaction was also run at rt and
reflux, but in none of the cases was the desired product obtained. em-CPBA= meta-chloroperbenzoic acid. fCAN = ceric ammonium nitrate.
Scheme 4: Attempted oxidative deacylation.
Scheme 5: Attempted desulfurization with Raney nickel.
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Beilstein J. Org. Chem. 2017, 13, 762–767.
9. Jepsen, T. H.; Jensen, A. A.; Lund, M. H.; Glibstrup, E.;
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was removed, degradation occurred.
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Conclusion
In conclusion, we have developed a desymmetrizing intramo-
lecular conjugate addition of a tethered dithiane moiety to para-
cresol-derived cyclohexadienones. The substrates are easily
accessible from cheap starting materials and the reaction
provides functionalized bicyclic lactones as a single diastereo-
mer. The products of the reaction were able to undergo dia-
stereoselective convex-facial additions. The carbonyl deprotec-
tion was unsuccessful and we hope that our efforts can serve as
a cautionary tale for future synthetic planning involving related
structures.
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Supporting Information File 1
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Experimental procedures, characterization data and copies
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[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-13-75-S1.pdf]
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The project described was supported by Award R35 GM118055
from the National Institute of General Medical Sciences. X-ray
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