K. C. Nicolaou et al.
1). These results were accompanied by another equally re-
vealing observation. Thus, as a consequence of their invert-
ed cyclobutane configuration, the new biyouyanagin struc-
tures 12 and 14 exhibited drastic chemical shift changes as
compared to biyouyanagin A (1), as shown in Figure 3. Spe-
cifically, the lone olefinic proton on the vinyl group (H-2)
was found at a considerably lower d value (i.e., 12: d=
6.10 ppm; 14: d=6.10 ppm) than that of 1 (d=5.29 ppm)
and its 24-epi-diastereoisomer (24-epi-1) (d=5.32 ppm), ap-
parently due to the prevailing anisotropic effect in these “in-
verted” structures. Indeed, manual molecular models clearly
show the H-2 proton residing in the vicinity opposite the
phenyl group within structures 12 and 14 (see structures in
Figure 3). At this stage we took note that natural biyouyana-
gin B exhibited a similar downfield shift (d=6.07 ppm, see
1H NMR spectrum for 2, Figure 3). The other two biyouya-
nagins (i.e., 11 and 13) exhibited the same downfield shift
(11: d=5.06 ppm and 13: d=5.06 ppm, see Figure 3).
Having synthesized all four possible major biyouyanagins
from hyperolactone C (4) [that is, 1 (see Figure 1b),[3] 24-
epi-1 (see Scheme 3b),[3] 11–14 (see Scheme 3a)] and not
having found the correct structure of biyouyanagin B, and in
the face of our newly gathered intelligence regarding the di-
1
agnostic H NMR chemical shift for H-2, we decided to re-
examine the original photocycloaddition of hyperolactone C
(4) with ent-zingiberene (ent-3) from which we previously
isolated biyouyanagin A (1) (see Scheme 4).[3] Our intention
was to look more carefully for minor biyouyanagin-like
1
products. Indeed, upon careful examination of the H NMR
spectrum of the crude cycloaddition product of the reaction
of ent-3 and 4, we detected, in addition to 1, two new minor
biyouyanagin-like components whose downfield 1H NMR
signals for H-2 were encouraging. Scaling up the reaction
and employing a larger excess of the zingiberene partner
(ent-3:4 ca. 20:1), we were able to isolate milligram quanti-
ties of the two new biyouyanagins and elucidate their struc-
tures. Delightfully, one of them proved to be identical in all
respects with the long sought-after biyouyanagin B (2, 3%
yield). The other one possessed structure 15 (2% yield) and
was named biyouyanagin C. These assignments were based
on spectroscopic analysis (see Figure 4 for NOEs of 2 and
15), and were consistent with their relatively low-field
1H NMR signals for H-2 (2: d=6.07 ppm and 15: d=
6.07 ppm, see Figure 3). Upon prolonged standing in a mix-
ture of CH2Cl2/hexanes, biyouyanagin B (2) crystallized as
colorless needles (m.p. 125–1268C). Its X-ray crystallograph-
ic analysis (see ORTEP representation, Figure 5)[6] con-
firmed its structure unambiguously, ending the quest for the
true structure of biyouyanagin B.
Scheme 3. a) [2+2] Photocycloaddition of zingiberene (3) and 7-epi-zin-
giberene (7) with hyperolactone C (4). Reagents and conditions:
(1.0 equiv), 3, 7 (4.0 equiv), 2’-acetonaphthone (1.0 equiv), CH2Cl2, hn,
320 nm, 58C, 12 h; 54% yield (11); 4% yield (12); 37% yield (13); and
19% yield (14). b) [2+2] Photocycloaddition of ent-7-epi-zingiberene
(ent-7) with hyperolactone C (4).[3]
4
Scheme 3)[3] (where the Ph and C-3 are found on the same
side of the central five-membered ring).
After eliminating the four structures derived from 4-epi-
hyperolactone (5) and zingiberenes 3, ent-3, 7, and ent-7, we
reasoned that biyouyanagin B is most likely derived from
naturally occurring hyperolactone C (4) in partnership with
one of the remaining zingiberenes (i.e., 3 and 7). We, there-
fore, set out to combine separately 3 and 7[3] with 4.[3] As
shown in Scheme 3a, each cycloaddition gave two new
biyouyanagins (11, 54% and 12, 4%; 13, 37% and 14, 19%
yield, respectively) as major products, but much to our sur-
prise, again, neither proved to represent the elusive biyouya-
nagin B structure. Interestingly, however, two of the prod-
ucts (i.e., 12 and 14) derived from hyperolactone C (4) pos-
sessed the “inverted” cyclobutane configuration as com-
pared to the other two (i.e., 11 and 13), which resembled
biyouyanagin A (1) and its 24-epi-diastereoisomer (24-epi-
Although there is no conclusive evidence to exclude en-
zymes participating in the [2+2] cycloaddition step of the
biosynthesis of the biyouyanagins, they may be formed, as
numerous other secondary metabolites, through spontane-
ous hetero- or self-assembling mechanisms.[7] It was, there-
fore, of interest to explore the reactivity of the various com-
ponents of the biyouyanagin forming [2+2] photocycloaddi-
tion. In a combinatorial experiment we employed all four
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Chem. Eur. J. 2010, 16, 7678 – 7682