Structure-activity relationships involving the aromatic ring of the selective PDE IV inhibitor, rolipram (l), are discussed. Rolipram [(R, S)-4-(3-cyclopentoxy-4-methoxyphenyl)-2-pyrrolidinone] (1) is a selective inhibitor of cAMP-specific phosphodiesterase type IV (PDE IV).¹ Due to the wide interest in developing therapeutic agents for chronic inflammatory diseases through selective inhibition of PDE IV, there has been a considerable medicinal chemistry effort aimed at improving the potency of rolipram. A number of groups have reported the synthesis of PDE IV inhibitors derived from rolipram, in which the modifications were made to the pyrrolidinone ring.² Less attention, however, has been directed to modifications of the catechol ring.³ In this Letter we report structure-activity relationships of rolipram derivatives in which the aromatic ring has been modified. The exact nature in which rolipram binds to PDE IV is unclear due to a lack of structural data on the enzyme. Our primary goal therefore was to prepare derivatives of the catechol that would help us ascertain the nature in which the catechol ethers interact with the enzyme. The methylenedioxy- and ethylenedioxyphenyl derivatives, 2⁴ and 3,⁵ respectively, were prepared and found to be considerably less potent inhibitors of PDE IV than rolipram itself (see Table). A similar loss in potency was observed with the derived cyclohexylidene acetal⁴.⁶ We reasoned that the loss in activity observed with compounds 2-4 perhaps stemmed from an improper orientation of the electron lone pairs associated with the oxygen atoms of the catechol. If one or both of these oxygens are involved in hydrogen bonding to an amino acid residue on the enzyme surface, clearly then the spatial geometry of these electron lone pairs is critical.⁷ The cyclic framework holding the catechol subunit of 2-4 forces the general dipoles created by the lone pairs to be directed away from each other (Figure 1a). However, it is possible that the optimum binding mode for rolipram and related derivatives is such that these dipoles are generally oriented in the same direction (Fig 1b). Indeed, the minimum energy conformation of rolipram (Fig 1c) has the two alkyl groups of the catechol ethers oriented similarly to the depiction in Figure 1b. In order to test this hypothesis we required aromatic derivatives of rolipram wherein the oxygen lone pairs are constrained to meet the geometric arrangement shown in Figure 1b. If, in fact, the orientation in Fig. 1b is required one might expect that compounds possessing this arrangement would exhibit equal or improved potency versus rolipram for inhibiting PDE IV. By simply anchoring the methoxy group at the 4-position to the ring we designed the benzofuran 5, dihydrobenzofuran 6, and dihydrobenzopyran 7, Each of these compounds meets the generalized criterion set forth by Fig. 1b. At the outset of this work, however, we were not certain of the enzyme's tolerance for the added substituent on the aromatic group in 5-7. Additionally, the effect of lowering the basicity of one of the catechol oxygens when going to a benzofuran (i.e., sp³-hybridization in 1; sp²-hybridization in 5) was unknown. The synthesis of benzofuran 5 is shown in Scheme 1. Knoevenagel condensation between 3-furaldehyde (8) and diethyl malonate provided the unsaturated diester 9. DIBAL reduction of 9 (toluene, -78 °C), followed by acetylation of the derived diol afforded 10. Direct formation of the benzofuran ring from 10 was then accomplished using a palladium-catalyzed cyclocarbonylation reaction recently reported by Hidai and coworkers.⁹ Thus, treatment of 10 with Pd(PPh₃)₂Cl₂ (5 mol %) and triethylamine in toluene under an atmosphere of CO (1000 psi, 175 °C, 11 hr) provided, after silica gel chromatography, benzofuran 11 in 45% yield. Saponification of the acetate esters (LiOH, H₂O/MeOH) provided an intermediate diol, which was selectively cyclopentylated (cyclopentyl bromide, K₂CO₃, DMF) to give the benzofuran alcohol 12 in 74% yield from 11. Oxidation of the benzylic alcohol with MnO₂ then delivered aldehyde 13 in 39% yield. With 13 in hand we followed a previously established three-step sequence to append the pyrrolidinone ring.⁸ᵃ Thus, Homer-Emmons homologation of 13 provided an unsaturated ester, which upon treatment with CH₃NO₂ and tetramethylguanidine (TMG), afforded an intermediate nitroester resulting from conjugate addition of CH₃NO₂. Catalytic reduction of the nitro group afforded an intermediate amino ester, which undergoes spontaneous cyclization to provide the rolipram derivative 5 in an overall yield of 53% from 13.¹⁰ The syntheses of rolipram derivatives 6 and 7 began from dihydrobenzofuran 14 and dihydrobenzopyran 15, respectively, and is shown below in Scheme 2.¹¹ Thus, O-alkylation (cyclopentyl bromide, K₂CO₃, DMF) provided the aldehydes 16 and 17, which were then converted to 6 and 7 by the same three-step sequence described above in the synthesis of 5.¹² Test compounds were measured both for their ability to inhibit the catalytic activity of PDE IV (Kᵢ)¹³ and for their ability to bind to the rolipram high-affinity binding site (Kd).¹⁴,¹⁵ The data are shown in the Table. As mentioned above, compounds 2-4 were considerably less potent inhibitors of PDE IV than (+)-rolipram. Consistent with our hypothesis, activity is restored in the dihydrobenzofuran derivative 6 and dihydrobenzopyran derivative 7, which are similar in their ability to inhibit PDE IV, Kᵢ = 3.69 and 1.17 μM, respectively. Perhaps most noteworthy is the benzofuran analog 5, whose PDE IV inhibitory activity (Kᵢ = 0.26 μM) is nearly equal to rolipram. It appears that lowering the basicity of the ether oxygen does not necessarily have a deleterious effect on enzyme inhibition. Within the framework of our hydrogen-bonding hypothesis, it is then reasonable to suggest that the in-plane sp²-hybridized lone pair of the benzofuran oxygen is better oriented to accept a hydrogen bond than its counterpart, out-of-plane sp³-hybridized lone pair in the dihydrobenzofuran 6.¹⁶ This being the case, the enhanced activity of the dihydrobenzopyran 7, relative to 6, is easily understood due to the greater flexibility of a six-membered ring versus a five-membered ring. This added flexibility in 7 allows the oxygen to position itself more favorably for hydrogen bonding. Finally, the rolipram binding data for this set of compounds parallel the enzyme inhibition data. Indeed, like rolipram itself, which has a Kd that is approximately 10-fold lower than its Kᵢ, compounds 5 and 7 display significantly greater binding affinity than PDE IV inhibition. In summary, we have presented the PDE IV inhibitory activity of a number of rolipram derivatives, which were designed to explore the nature in which the catechol ether moiety interacts with the enzyme. Our goal was to understand the optimum spatial arrangement of the catechol ethers, not simply to assess the steric tolerance at each oxygen. We have provided evidence that the spatial arrangement of the oxygen lone pairs of the catechol ethers are critically important with respect to the PDE IV inhibitory properties of rolipram and its derivatives. The ability to design potent inhibitors of PDE IV, which are based on rolipram, should be facilitated by the present study.