The SPCO's world premiere of a Libby Larsen work is an "exemplary music of ideas."

"Bach and his progeny" is the unstated topic of this week's St. Paul Chamber Orchestra program, led by Nicholas McGegan with his signature ebullience. The center of interest is the premiere of Libby Larsen's "Evening in the Palace of Reason" for string quartet and string orchestra. Commissioned to honor St. Paul lawyer Lowell Noteboom, the five-movement work is a contrapuntal meditation on the interconnected cultural revolutions of Bach's day and of ours; it takes its initial impetus (and its title) from a book by James Gaines that teases out the import of Bach's 1747 encounter with Frederick the Great of Prussia.

Witty, discursive, full of unexpected juxtapositions, "Evening" blends a host of musical ingredients: Along with two borrowed Bach themes (worked to near-exhaustion) and the "galant" style favored by Frederick, I hear Britten, Ives, jazz and more. But this complex stew is sagely seasoned, managing not to deter listeners whose taste for spice is circumscribed.

Larsen's string writing is accomplished; her textures, often dense, are never opaque. Though at moments it might have swung a bit more, Friday's rendition was compelling, with outstanding contributions from the solo foursome: Dale Barltrop, Nina Tso-Ning Fan, Sabina Thatcher, Ronald Thomas. An exemplary music of ideas, "Evening" is the product of searching thought -- and it makes one think.

Bach's Orchestral Suite No. 1, essentially a succession of French dances, lets the old man speak for himself. At times I missed the timbre of period instruments, oboes and bassoon in particular. But McGegan excels at infusing performances on conventional instruments with a period sensibility. And when in his groove he doesn't beat rhythms so much as live them.

Felix Mendelssohn was one of the great composers of his era, but his most important contribution to music may well have been his 1829 Berlin performance of Bach's then-obscure "St. Matthew Passion" -- arguably the foundation of Bach's modern reputation. In the same year, in London, Mendelssohn conducted his Symphony No. 1, its original Minuet replaced by an orchestration of the feathery Scherzo from his string Octet.

It's this London version that McGegan chose for his closer. His affection for the piece was evident, but even he can't convince me that its superficially stormy outer movements, which fail to conceal a lack of real tension, amount to much more than an exercise in mimicry.

Larry Fuchsberg writes frequently about music.