You should probably be using the Homo.sapiens package, which wraps up a bunch of annotation packages to make this easier. Also, if you just want one row per transcript, you should use mapIds rather than select. Something like this seems appropriate:
> library(Homo.sapiens)
> keys <- keys(Homo.sapiens, "TXNAME")[1:500] ## let's not get crazy here
> colstoget <- c("SYMBOL","ENTREZID","ACCNUM","REFSEQ","TXCHROM","TXSTART","TXEND")
> z1 <- as.data.frame(lapply(colstoget, function(x) mapIds(Homo.sapiens, keys, x, "TXNAME")))
> names(z1) <- colstoget
> head(z1)
SYMBOL ENTREZID ACCNUM REFSEQ TXCHROM TXSTART TXEND
uc001aaa.3 DDX11L1 100287102 AM992871 NR_046018 chr1 11874 14409
uc010nxq.1 DDX11L1 100287102 AM992871 NR_046018 chr1 11874 14409
uc010nxr.1 DDX11L1 100287102 AM992871 NR_046018 chr1 11874 14409
uc001aal.1 OR4F5 79501 BAC05820 NM_001005484 chr1 69091 70008
uc001aaq.2 <NA> <NA> <NA> <NA> chr1 321084 321115
uc001aar.2 <NA> <NA> <NA> <NA> chr1 321146 321207
But this only takes the first of any duplicate values, which is sort of naive and stuff. If you can handle ambiguity, you can do something slightly different.
> z2 <- as(lapply(colstoget, function(x) mapIds(Homo.sapiens, keys, x, "TXNAME", multiVals = "CharacterList")), "DataFrame")
> names(z2) <- colstoget
> z2
DataFrame with 500 rows and 7 columns
SYMBOL ENTREZID
<CharacterList> <CharacterList>
uc001aaa.3 DDX11L1 100287102
uc010nxq.1 DDX11L1 100287102
uc010nxr.1 DDX11L1 100287102
uc001aal.1 OR4F5 79501
uc001aaq.2 NA NA
... ... ...
uc001bap.3 ARHGEF10L 55160
uc010ocr.1 ARHGEF10L 55160
uc001baq.3 ARHGEF10L 55160
uc001bar.3 ARHGEF10L 55160
uc010ocs.2 ARHGEF10L 55160
ACCNUM
<CharacterList>
uc001aaa.3 AM992871,BC032353,BC070227,...
uc010nxq.1 AM992871,BC032353,BC070227,...
uc010nxr.1 AM992871,BC032353,BC070227,...
uc001aal.1 BAC05820,NM_001005484,NP_001005484,...
uc001aaq.2 NA
... ...
uc001bap.3 AAH65561,AAH80596,AAI17172,...
uc010ocr.1 AAH65561,AAH80596,AAI17172,...
uc001baq.3 AAH65561,AAH80596,AAI17172,...
uc001bar.3 AAH65561,AAH80596,AAI17172,...
uc010ocs.2 AAH65561,AAH80596,AAI17172,...
REFSEQ TXCHROM
<CharacterList> <CharacterList>
uc001aaa.3 NR_046018 chr1
uc010nxq.1 NR_046018 chr1
uc010nxr.1 NR_046018 chr1
uc001aal.1 NM_001005484,NP_001005484 chr1
uc001aaq.2 NA chr1
... ... ...
uc001bap.3 NM_001011722,NM_018125,NP_001011722,... chr1
uc010ocr.1 NM_001011722,NM_018125,NP_001011722,... chr1
uc001baq.3 NM_001011722,NM_018125,NP_001011722,... chr1
uc001bar.3 NM_001011722,NM_018125,NP_001011722,... chr1
uc010ocs.2 NM_001011722,NM_018125,NP_001011722,... chr1
TXSTART TXEND
<CharacterList> <CharacterList>
uc001aaa.3 11874 14409
uc010nxq.1 11874 14409
uc010nxr.1 11874 14409
uc001aal.1 69091 70008
uc001aaq.2 321084 321115
... ... ...
uc001bap.3 17907048 18024370
uc010ocr.1 17914911 17966476
uc001baq.3 17941583 18024370
uc001bar.3 17944811 18024370
uc010ocs.2 17944811 18024370